I've heard this word a few times now. It means when someone appears threatening but does not actually back it up with feats of strength or bravado. They are threatposing. Posing to be threatening but that's about it.
"I went by Darby's Bar yesterday and there was a bouncer there threatposing at the door."
"Yeah, I know who you mean. That's Gill. Beneath all that toughness he's a real creampuff."