The front leg of any animal that walks on all fours.
Since my dog Kodiak doesn't know how to pee with his leg up, he's always pissing on his arm-legs.
When your work colleague asks for something that you already sent them, and you just hit forward on the original email with no note.
Sanjay thought he was catching me being late on a project, when really I already sent him my part two weeks ago. So he got a fuck you forward when I sent it ... yet again.