Today, the word nuclear might conjure images of exploding warheads, but it literally just means "of or pertaining to the nucleus", because that is what's being split, after all. When it was first borrowed into the English language in 1668, nucleus referred specifically to the main part of comets or meteors, and that was later broadened to central masses in general. The word is from Latin, where it meant "kernel". That's from nucula, which meant "little nut" and was the diminutive of nux (ultimately from Proto-Indo-European kneu, "nut"). The pronunciation nucular, which has been used by four U.S. presidents and several nuclear scientists, is now included in the Merriam-Webster dictionary. Hypotheses for why people say that vary: linguist Stephen Pinker thinks the u was inserted because it's easier for English speakers to say that than the accepted pronunciation, but others say it's because people think of nukes and then attach the -cular suffix from words like molecular. There's also an interesting theory that the politicians who say it but know better deliberately do so to sound folksy.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.