Nick Young -- Trolled By Gilbert Arenas ... 'YOU GOT DUMPED!'

Gilbert Arenas is throwing some major salt in Nick Young's wounds -- clowning the guy for getting dumped by Iggy Azalea ... and he ain't pullin' any punches.

We THOUGHT the two were friends -- especially when Gilbert told us he had planned on going to Nick's bachelor party a few months back. But Nick wasn't having it and told him he wasn't invited.

Fast forward to Monday night ... after Gilbert found out Iggy broke up with Nick ... and Agent Zero unloaded on his former Washington Wizards teammate on the Gram.

"@swaggyp1 i guess u wont be invited to YO #bachelorparty either 😂😂😂😂#gotdumped."

"BUT dont worry n***a i got you...#bachleorparty just turned into #TheBreakUpParty at #Samsstripclub we about to be in there with a pocket full of (1's) with no draws on."

He went on to wax poetically about how he hopes Nick engages in unprotected coitus with several ladies ... and then explains why he hates condoms. You can click here for the whole thing.