Green’s Dictionary of Slang

blow it out your ass! excl.

also blow it! blow it out! blow it out of your bag! blow it out your barracks bag! …B-bag! …butt! …footlocker!
[SE blow + ass n. (2)/butt n.1 (1a)]

[1940s+] (orig. US milit.) a general excl. of derision, contempt or dismissal of the previous speaker’s statement.