Tasmad Rudrah ity akhyayate (That's the Rudra I'm talking about here)

Lets see an example of how Sanskrit word roots pervade languages, cultures and continents. Rudra is a Rigvedic deity, associated with wind and storm. The theonym [1] Shiva originates as an epithet of Rudra, the adjective shiva ("kind") being used euphemistically one who is also feared as the ghora ("terrible"). Usage of the epithet shiva came to exceed the original theonym by the post-Vedic period in the Puranas and the Epics. 

Let us analyze the roots of the word Rudra. The epithet is ambiguous not by coincidence but by deliberate design, a prevalent practice in naming Indian gods (nothing is ambiguous by chance in Sanskrit). In popular parlance, the name has been primarily translated as "the roarer". A more detailed etymological derivation is provided by Sayan. Acharya Sayan (1372-1444 A.D.) wrote the most "accessible" commentaries on all the four Veda Samhitas in modern Sanskrit language (he is also the oldest available commentary on the Atharvaveda). 

Rodayati sarvam antakale iti Rudrah ||
yadva rut samsarakhyam dukham |
tad dravayaty apagamayati vinasayati iti Rudrah ||
yadva rutah sabda-rupah upanishadaha |
tabhir druyate gamyate pratipadyate iti Rudrah ||
yadva rut sabdatmika vani tat-pratipadyatma-vidyava |
tam upasakebhyo rati dadati iti Rudrah ||
yadva runnadhi avrinoti iti rud andhakaradi |
tad drinati vidarayati iti Rudrah ||
yadva kadachid devasura-sangramegny-atmako |
Rudro devair nikshiptam dhanam apahritya nigarat ||
asuran jitva devah enam anvishya drishtva dhanam apaharam |
tadanim arudat |
tasmad Rudrah ity akhyayate ||

Essentially the word Rudra can be decomposed into two words - rud and dra. Each of these root words have multiple meanings, and by permuting over those meanings the multi-way pun rudra is formed - with meanings varying from the kind to the terrible. By design, all by design. This is so for most words in Sanskrit, whatever their simple (popular)-meaning (roodh-artha) be, their yoga-artha ("combination-meaning", with these combination rules defined according to a meta-language as interpreted by Panini in his Ashta-adhyayi) can be highly malleable.

Let us revisit a few of Sayan's explanation in English. Also pay close attention to how English and Sanskrit words that sound similar, also mean the same. 

Rodayati sarvam antakale iti Rudrah ||

 "One who makes (rodayati) all (sarvam) cry for mercy (rodayati) at the end-time (anta-kale) of the universe, He is Rudrah."

rud (ruth) here means crying (for mercy of course). In this form, rud forms the root for the Sanskrit word roodan (the process of crying), also rona in Hindi and through its Latin derivative reuthe, the root for the English word rue. Also observe the similarity of words end and anta, English and Sanskrit forms of the same word.rud (ruth) also means mercy, and preserves that meaning in the Latin word reuthe, from which has been derived the word ruthless (mercy-less).

yadva rut samsarakhyam dukham |tad dravayaty apagamayati vinasayati iti Rudrah ||

"Rut is the suffering called the world (samsara), he who drives (dravayati) that (tad) away (apa-gamayati) and destroys (vinasayati) it is Rudrah."

Rut here stands for suffering (a word is allowed multiple meaning since the meta-language behind Sanskrit concerns itself only with the philosophical content of the word, which does not change here). dravayati is the verb form of the root dra (meaning "flow"). dravayati means "making flow". The root dra also forms the word dravya (one that flows) the Sanskrit word for liquids, and also, quite interestingly, money. Seems like they realized long back that money always flows and remains with none for too long (as it should to keep the economy working, a concept lost on those who are driving the Indian economy to ground by shamelessly (and to much dismay of Chidu) hoarding gold and sucking monetary liquidity out of economic circulation). dra is also the grand-daddy of the Proto-Germainic root dreug, from where have been derived words like drain (flow out), drought (flowed out) and dry. It is also indirectly related to the word drive (make flow). Surprised? You ain't seen nothing yet!

The word tad morphs into the Proto-German that, and is adopted into English that way.

yadva rutah sabda-rupah upanishadaha |tabhir druyate gamyate pratipadyate iti Rudrah ||

"rut in plural means Upanishads, by which He is reached (gamyate), expounded (druyate) by those very (tabhir) Upanishads, thus Rudrah."

dru is related to dra above and stands for explain-ation. What is an explaination but a flow of knowledge.

yadva runnadhi avrinoti iti rud andhakaradi |

tad drinati vidarayati iti Rudrah ||

"Rudh means to cover, or shut. And so rut means something that has been shut, or darkness. He rends (drinati) that darkness, so he is designated Rudrah." [2]

In this sense rut is also the root of the Sanskrit word raatri (the period of darkness) and its colloquial Hindi form - raat.  

The extra etymolgical content in the rest of the shlokas related to this post isn't much, so I will skip them. 

Words propagate across time and space, much like living beings - mating, multiplying, adapting themselves. Just like genes they self-preserve at any cost. To understand this we must look closely for cognates [3], words with the same etymological root. Sometimes the same word is pronounced differently in different cultures (Katrina and Catherine). As words travel their pronunciation gets obfuscated from the original master languages (Sanskrit, Greek, Latin). They become what in Sanskrit is called an apa-bhramsh, literally going away (apa) to fall (bhramsh) [4]. Prakrit, Awadhi, Urdu and Hindi are thus apabhramshams of Sanskrit, (and so are Bengali, Marathi, Gujarati, Odiya, Assamese etc.) replacing hard nosed rules of Sanskrit with digestible simplifications and doing away with the Sanskrit word-factory with a fixed vocabulary borrowed from popular Sanskrit and Persian to various degrees. One way to spot these words is to look for colloquial pronunciation loop-holes, for example - tra is often pronounced tar (Katrina in the East but Catherine in the West, matri in Sanskrit becomes mother in English), dra as der (Rajendra in India but Alexander in Macedonia).   

1. theonym: Greek theos (god) and onym (name) from Sanskrit namah (meaning salutation, kaar is form, so namaskaaram hence is literally "I salute your form"). Theonyms are useful in understanding the connections of Indo-European languages (Sanskrit-Greek-Latin).

2. Rudh-artha: simple meaning. Here rudh means dark, covered because the original meaning (yoga-artha) of the word as        determined by the rules of grammar is in the dark.

3. Cognate: from Latin cog (blood) - natus (relative, also natal, native, nation).

4. Our closest relatives.

        bhra - root word of Sanskrit bhrata (brother, oops!), bhai in Hindi. 

        pitr - root word, from this is derived Sankrit/Hindi pita (father), its English derivative is Father, we have the word patri-cide (father-murder) as our witness. 

        matri - root for Sanskrit mata (mother) and English mother, mom, or in James Bond's and my contact list - M.