Ignore the erotic definitions made by horny teens below, but a blast from the past is something that was made from the past, but its very amazing, even to the present day.

the act of thinking of (or popping up) previous sexual encounters while masturbating

a blast from the past is when you hook up with someone you previously dated or had history with and bust a nut on her.

The act of giving your grandfather a blowjob and having him ejaculate in your mouth.

The act of ejaculating in ones anus then having that person fart a cum filled mess on to your face.

Giving your grandpa a blowjob and having him shoot his load in your mouth.

Food and drink induced diarrhea after a night of binging or a bender.