B.F.o.t.D. - The bodily function you have to do because your body doesn't

The bodily function you have to do because your body doesn't care about your plans.

I had a date and my B. F. o. t. D. ruined everything.

I was in the middle of a speech and my B. F. o. t. D. interrupted me.

I was trying to sleep and my B. F. o. t. D. woke me up.