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.
A person or group of people, who take an activity so seriously and or literally, that they squeeze every last drop of joy out of it.
Can be appropriately applied to cyclists who spend 10K+ on their bikes, clearly don't enjoy what they're doing and live for looking down on those with 'lesser' equipment.
"Did you see those Joe Serious bellends shaking their heads and tutting?"