A fun sponge in the military is a socially inadequate, humourless & deeply unpopular person who, through a sense of bitter loathing & revenge, sucks all the fun out of everything. Normally calls senior officers ‘sir’ on twitter.
Dude: why don’t we show our human side & engage online?
Military Fun Sponge: all this social media stuff is only a passing fad & has to be boring.
That person in your group of friends who doesn't have a licence and seems never likely to get one. This means either you or your other licenced friends, if going on an outing together have to drive them around ALL the time as they don't have a licence which means they are unable to return the favour and drive you for once.
George: Kyle totally pisses me off, why can't he stop being a car sponge and get a licence
Stu: Yeah tell me about it, the other day I had to drive that car sponge to the bank and I had to wait over an hour for him