We get our word sinister ("evil") from Middle English sinestre, which meant something more along the lines of "unfavorable", and that is from Old French sinistra, which meant "left". There's a lot to explain here; the transition from "left" to kinda "unlucky" occurred because the left hand is associated with clumsiness and abnormality among right-handed people, who got to name the word, and the meaning merely amplified to "evil" over the centuries, to get to today's words. Anyway, sinistra comes from Latin, where the word once again takes the form of sinister (I know, it was much ado about nothing). The suffix is not -er here; this is not Germanic. The suffix is -ter, formed as a counterpart to dexter (we'll get to that tomorrow), and the root is a mystery to etymologists, but we theorize that it goes back to a Proto-Indo-European word, because it may have a Sanskrit cognate.
Hello! I'm Adam Aleksic. This year, I graduated from Harvard University with a degree in Government and Linguistics. There, I co-founded the Harvard Undergraduate Linguistics Society and wrote a thesis on Serbo-Croatian language policy, magna cum laude. In addition to etymology, I also really enjoy philosophy, trivia, vexillology, geography, board games, conlanging, art history, and law.