the process of ejaculating on the buttocks of your partner, then spread the cum all over the buttocks.
hit it doggy style (in either hole), then ejaculate on her/his ass. take your penis to butter the bagel, then reinsert into either hole.
guy to girl: yo, let me butter your bagel
girl to guy: can you butter my bagel, please?
dude#1 to dude#2 about a girl: dude, i buttered that bagel last night