Literally "Thigh-beater" means a joke and is named after the reaction of hitting ones thigh with ones hand while laughing, often used ironically for a really bad joke.
Adjective, "fromthenceran". "Some fromthenceran guy came into my bakery and tried to tell me how to bake". Something like "uninvited, unqualified, obnoxious, without renown", but also "plain common". Not necessarily a person.