to laugh at your own joke whilst you're still attempting to tell it, resulting in a mumbly laugh (or lumble) escaping your lips.
this usually happens during particularly bad jokes rather than particularly good ones, as the embarassment as you realise just how poor your joke is triggers a nervous laugh
people who lumble tend to find humour in everything they see (a mobile library, small children, a lamppost etc.) and so are quite often heard lumbling their way through some half-assed joke that they really didn't think through
it's quite funny, i just saw a mobile libray, huh huh, and that's like... a library, but, hur hur, it's got wheels... heh heh, it's like, a library on wheels. huh. huh. *awkward pause*
... i think i just heard a lumble