I hear the word bum for buttocks every once in awhile, usually older generations.... except phrase "cute as a baby's bum" which transcends generations.
When I hear "bum" I sometimes think of "bottom" first as that is where my mind oft dwells ... a lot depends on context ... when I first saw the B.U.M. T-shirts I wondered what the letters stood for (still don't know) and then wondered why anyone would use the word BUM as a name thereby advertising that the product is bum (i.e. bad, inferior, worthless.) Then I bethought "Ah Ha! Corporate thinking" and that explained it all!
Meanwhile, dear Bean, in answer to your post : if I wore a shirt that said T. I. T., you would all giggle first and ask questions later, wouldn't you?
No, actually I would extrapolate Tit to Titmouse as the dictionary says tit is a short form of titmouse and probably (smart a** that I am) sweetly ask you) "Are you an ornithologist?"
How-some-ever, if you were wearing a shirt emblazoned with TEAT .... Well! (note restraint of impulse to use blush emoticon)