A century and a half ago, there was a television personality named Art Linkletter (who, by the way, turned down Walt Disney's generous offer for Art to commercially develop the land surrounding a little amusement park Walt was planning to build in the orange groves of out-lying Las Angeles). Linkletter had a schtick called "Kids Say the Darndest Things". Art asked one little shaver how many people lived in his home. The tyke answered, "One adult and one adultress." This simple statement (whether fact or faux pas has never been investigated) riveted my attention for years. What is the difference between adult and adultery and why is it so desirable to become one and not the other?
Throughout BackWords we're going to deal with many prefixes and suffixes, most of which came to us from Latin and Greek. Our first prefix is one of the most common: ad- meaning "to". The second part of adult is, obviously, ult - does that ring a bell? ult derives from an IE (remember, Indo-European) word for "beyond". You see it in words like "ultimate", "ulterior", and even "alter" and "alternate". An adult is "to beyond" or better, has grown to (an age) beyond (childhood). Similarly (in terms of meaning) an adulterer is someone who has gone to (someone) beyond (the marriage partner). We have here identical word parts with dramatically different meanings. That happens a lot!
BONUS WORD: Hidalgo is a given-name in Spanish cultures. See the al part of the word? It also derives from the idea of "beyond". The hid part was originally fid... that we see in in words like "fillial" and "affiliate" with a core meaning of "family". Hid, in this case means son. The al part in this word means "beyond you and me". Hidalgo mean "the son who is beyond (belonging to) me or (belonging to) you, but is the son of all of us." We have a term "favorite son" that has this same idea - because a popular or accomplished person is from our home town, we can all claim ownership of him.