Pete Davidson, Machine Gun Kelly Strip Down to Undies, Joke About Their Junk

Pete Davidson and his pal Machine Gun Kelly are talking about the potential elephant in the room -- namely how they landed smoke show GFs -- and they're doing it in their undies.

The duo stripped down Monday night during a takeover of Calvin Klein's IG live -- Pete's got a deal with CK -- and the buddies, who are famously dating Kim Kardashian and Megan Fox, brought penis jokes aplenty.

While still fully clothed, PD joked he'd have been willing to kiss MGK on cam or even touch penises if CK upped the budget for their appearance -- and then they decided to flash their tightie whiteys.

After dropping trou, Pete checked out MGK's manhood and remarked, "Nice stuff, by the way, I get it now. Good for you. Good for you, dog." Kelly replied, "I would’ve put an extra 2 socks in there and blew the world’s mind."

Pete was more self-deprecating, saying ... "I’m not really a grower or a shower. It’s actually the same small and hard. It’s actually a scientific wonder."

Of course, the buddies are leaning into what everyone's been buzzing about -- how Pete, in particular, has hooked up with not only Kim, but Ariana Grande, Kate Beckinsale, and Kaia Gerber.

Ariana's famously made very flattering comments re: Pete's manhood ... and last night, MGK said Pete's name is now synonymous with BDE. Pete sheepishly said, "That’s insanely embarrassing, but it’s true."