Zac Efron brought his guns to Jimmy Kimmel last night. And he also brought his girlfriend, Sami Miro -- but it looks like she forgot something pretty important -- her pants!

What the heck is the girl wearing? That's a button-down SHIRT, Sami, not a dress!

Okay, so Zac's making the rounds, promoting his new film Dirty Grandpa, in which he actually got to share the screen with Robert DeNiro. Never thought you'd see that, did you?! The film follows A-type lawyer [Zac] who gets convinced into taking his grandfather [Robert DeNiro] on a road trip to Florida where babes and booze are plentiful. Wild times, folks!

Speaking of wild times, Zac got frisky backstage in hair and makeup, having girlfriend Sami take a shot of her him shirtless -- because, basically, he knows his fans DIE for this. And so do we ...