Hi, Bean. No, we don't use the word bum to mean anatomy, but it's easy to tell by the context alone which sense is meant by a speaker: we wouldn't, for ex., tell a child to
go sit on "a" bum! It would always take a possessive pronoun, I imagine.

Here's the link to the B.U.M. equipment website:
http://www.bumequipment.com/intro.html
It says the clothing company was started in a garage in Seattle, as a mild mockery of people who went around in clothes emblazoned with the name of their local gym.

By the way, I've printed out your explanation of time being a dimension and not having momentum.


EDIT: dag nab it, tsuwm!! Beat me out again!