I’ve often wondered if the words adult and adultery are related — you know, adultery is something adults do, that sort of thing. (An uncomfortable folk etymology at best.) I did a little digging in the OED and some Latin etymological dictionaries and found that, thank heavens, the two words have nothing to do with each other.
Adultery comes from the older French forms avouterie and adultère, respectively from the Latin words adulter (“adulterer”) and adulterium (“adultery”).
And where did adulter come from? Ad- (meaning “to”) and alter (“the other of two” — it’s the comparative form of alius, “another”). Add in a vowel shift from adalter to adulter and there you go.
So, an adulterer is someone who goes (ad usually implies movement) to “the other one,” to another — to someone who isn’t their spouse.
On the other hand, adult comes from the Latin adultus, which is the past participle of adolescere (“to grow up”). It means “grown up,” basically. Go figure.
(And adolescence is the condition of growing up, from the Latin adolescentia, which comes from the present participle, adolescentem.)