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Boisterous Weakling if they have nothing to back up their talk, Paper Tiger if they look like they could back it up but don't, Cowardly Lion if they actually could back it up but don't (they can overlap, but a character can be a paper tiger without being a cowardly lion and vice versa)
For completeness's sake, Why Did It Have to Be Snakes? for literal specific fears that might leave an otherwise badass character in this state.
Edited by Scorpion451
I think you're looking for Miles Gloriosus (similar to Boisterous Weakling, but focusing more on the Dirty Coward part). Cowardly Lion is more of an inversion of this phenomenon.
^ Miles Gloriosus is another that often overlaps, but it's more about them being a foolhardy braggart. That and Boisterous Weakling form sort of a trifecta with Fearless Fool, who talks big and then attempts to back it up with no regard to whether or not they actually can.
Cowardly Lion is weird in that it can be an inversion of what they describe, but it can also be exactly what they describe when the fear only kicks in during an actual crisis, but then they prove they can actually back the talk up when the chips are down. The trope namer for instance was very bad about talking tough and then running away, until he had someone other than himself to protect (namely Dorothy and co).
Edited by Scorpion451
Is there a trope for a character who acts tough and bold, but when threatened, even in the slightest, cowers into submission/obedience?