Thank you guys for your responses! I have more than once called out, loudly, to my kid, "Wear a wife-beater under that shirt!" And I just said, "Go put a wife-beater on, then eat your dinner, then put the nice shirt on." (He just got out of the shower and was getting dressed and I don't want him slopping food on his nice shirt.)
If I said "tank top" he would be confused. Tank tops are something you wear on the outside, or topside, of underwear. To me, a wife-beater is underwear, just like underpants are.
My kid is getting used to, but still can't believe, I call them that despite me explaining why they are called that.
Now, I want to know why the packaging calls them "A-Shirts".