Bruce Jenner -- I'm Letting It All Hang Out

Bruce Jenner can't be contained ... well, not by his shirt anyway.

B.J. hit up Starbucks Saturday afternoon in Calabasas after a round of golf, and got all crazy -- untucking his shirt on the way in for a more casual look.

As we previously reported ... Bruce is dating one of Kris Jenner's best friends named Ronda Kamihira -- and we're told his ex-wife has been crying every night over it.

Bruce and his ponytail ... seem okay with the single life.