If you're ambivalent, you feel indecisive about something, and that's literally what the etymology tells us, as well. So we get it straight from German Ambivalenz, which was coined by the Swiss psychologist Eugen Bleuler in 1910 and is meant to be capitalized. It was intended to describe the characteristic of people to see the strengths of both sides of the argument, and, in forming the word, Bleuler took the Latin prefix ambi-, meaning "both", and spliced it on top of the root valentia, meaning "strength". We've already covered ambi- in the ambidexterity post, so let's ignore that for now. Valentia, meanwhile, is also the etymon of valence (as in the electrons), because of a connection through a side definition of "capacity". This comes from the present active participle valens, from Proto-Italic waleo, from Proto-Indo-European hwelh, which still meant "to be strong", so not much change there. Usages of both ambivalence and ambivalent over time have been surprisingly uniform, increasing at basically the same rate and amount.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.