bite his tush?
It is actually used by the very crude.
I would have said "pinch" - as my not-religiously observant aunt attempts to perform whenever she's in my presence.
I'm a single BY girl in shidduchim and never used/nor have heard the expression 'bite his tush' when describing someone cute from infant to adult. Having said that-Shades, that's pretty amusing. I know lotsa people's aunts like that
“According to the statistics, in 1971 the chance of a 35-year-old woman in Israel being unmarried was 1 in 40. Today, at least one in four women of that age is unattached. The situation is the same with men.”SOURCE: The Jerusalem Post 6/27/2012 in the article "Israel is swamped with singles" by Arieh O’Sullivan.)