Kourtney Kardashian's letting it all hang out!

As she took son Mason to his music class, the mother-of-three exposed her Tommy Hilfiger undies above the waistline of her grey sweatpants. She took it a step further with a white cropped tee -- to make SURE we'd see her panties -- and topped it off with an Adidas sweatshirt and her usual Gucci sneakers. Talk about a label whore!

She looks awesome, though, and let's be honest, she's gotta keep it sexy if she wants to keep the attention of her younger supposed-boyfriend, 23-year-old French model, Younes Bendjima. The two were spotted out together Sunday night at the SLS hotel in Beverly Hills. Guess Bieber's over and she's on to her next boy toy!