A phrase referring to sex especially when one or both of the participants have large quantities of body fat. This can be derogatory, but is typically meant to be and taken as a joke.
Did you hear? Donna and Steve did the flumpa-lumpa last night!
A synonym for tired or exhausted. It might be accompanied by an overwhelming urge to do nothing more than chill out at home on the sofa / couch and do nothing,
I'm really looking forward to getting home. I feel really flumpy, I just want to chill out.
Or
I'm really flumpy today, and I don't feel like doing much at all
Or
I'm so flumpy, I don't feel ready to get out of bed!