Words change, both in form and in meaning. Take the changes that occurred when the Romance language developed out of Latin over the course of time. The Latin word for war is bellum. Yet, the daughter languages have: French gurre, Italian, Spanish, Portuguese guerra, and Romanian război. The first set of words are assumed to have been borrowed into Vulgar Latin from a Germanic language (Frankish, perhaps): the word is not recorded but is reconstructed as *werre 'commotion, confusion, disturbance' and is cognate with German Wirren; the German word for war is Krieg. (The Romanian word is borrowed from Russian разбой (razboj 'robbery' < raz- 'one another' + boj 'fight; battle'.) Why would the Latin word bellum be replaced by a loanword? Well, Latin had another word (an adjective) bellus, -a, -um, 'handsome, pretty'. (Another common Latin word for 'pretty' or 'beautiful' was pulcher.) So, it's thought that of these two homonyms, one remained in the daughter languages (French beau, belle, Italian bello, -a), but note Spanish lindo, -a, Romanian frumos, frumoasă) and one 'war' was replaced by a loanword. This sort of phenomenon has been noted before. The word for rooster in French is not from the Latin word gallus because that word in Old French sounded like the word for cat. What we call Latin was written and spoken as a living language for a period of about 800 years. During that time it changed. The two words bellus and bellum developed from two older Latin words: *duenelos (*dueno- being related to bonus, -a, -um, 'good')and duellum.
Another example: Latin latro, latronis. In early Latin (in Plautus [ca.254–184 BCE]), latro mean 'hired servant; mercenary soldier', but in later Latin another meaning started to develop 'brigand, pirate'. This latter sense appears in Ulpian [d. 228 CE]:
The sense of the word as it was inherited by the Romance languages: Italian ladro, -a, Spanish ladrón, -na, but French voleur, voleuse, Romanian hoţ. A diminutive of latro is latrunculus (cf. homunculus < homo 'man') 'pawn' (on a chess board a figuratively as in as a pawn of war.
Finally, a word about Latin hostis as used above. Originally, it meant 'foreigner, stranger', but it came to be specifically an 'enemy' (in the context of war). It is from the same PIE root *ghosti- as English guest and Greek ξενος (ksenos) 'foreigner', whence English xenophobic (link). In Late Latin, the word hostis became a mass noun, 'the enemy'. We inherited this meaning in host as in a 'multitude'.
[NB: an asterisk, *, is used to indicate that a form is not recorded, but is reconstructed.]
—Ceci n'est pas un seing.
Thanks for this, z. This was an excellent discussion on the chat on Saturday, though of course z has deepened the discussion here.