stink bomb; weasel's defensive odor
Literal meaning: the foul smell released by a weasel from its anal sacs when threatened. Also used for a stink bomb or similar prank item.
He brought a stink bomb into the classroom as a prank.
When attacked by an enemy, a weasel releases a foul odor.
last-ditch effort; final desperate move
Idiomatic use: a final, often desperate or petty, tactic employed when one is cornered or about to lose. Carries a nuance of a weak or spiteful parting shot.
As a final desperate act, he spread bad rumors about the company right before quitting.
The other party, after the negotiations broke down, threatened legal action as a last-ditch effort.
捨て台詞 is a parting shot or insult thrown as one leaves, focusing on words; 最後っ屁 can be any final desperate action, not just verbal.
Literally 'last fart', from 最後 (last) + 屁 (fart). The idiomatic sense derives from the image of a weasel releasing a foul smell as a final defense when cornered.