Some shit that a straight man flashes when he's hanging with bro's and begins to suspect that the bro's suspect he's gay. He will then proceed to make a comment--usually of a lewd sexual nature--about a woman to publicly confirm his straightness to the other bro's and assuage fears that he wants their body.
Male Cashier: "Man, that's a real nice scarf. I've never seen anyone match their scarf to their sweatshirt so well."
Bro: "Um.. thanks.."
*hot girl walks by*
Male Cashier: "Oh hey look, the college girls are back in town. Looks like some fine ass scenery!"
The Male Cashier realized that he sounded a bit too interested in the bro's fashion choice. He brought out his straight cred to fix this critical error.
One or more females associated with a guy or group of guys to create an image of heterosexuality.
Katie Holmes, Bar Refaeli, any chick with a pack of dudes
Jim: Hey, look at that hot chick riding in the back of that sausage wagon!
Tony: Yeah, that's their straight cred.
Dax begrudgingly brought his girlfriend with him to the lake as straight cred. He knew what a bunch of dudes in a giant boat shaped like a penis
would look like to onlookers.