A person who talks like they are great at a sport or activity but in reality he/she is below average and/or very bad at the respective sport or activity.
Carl tells everyone about how good a golfer he is, but the players at the club refer to him as a trunk slammer.
Derogatory term for a contractor who has minimal skill, experience and equipment. Trunk slammer implies the contractor works out of their car trunk, with no office or licenses and they will likely be difficult to find if the client is not happy with the work.
My neighbor hired a trunk slammer to build his house deck, and now we are laughing at the unfinished disaster that he spent so much money on.