Basement tits are the mini rolls underneath your breasts/chest muscles you get leading up to the holidays from all the over-excessive eating that is done during this time period.
They usually lie dormant until a family member or close friend points them out.
Basement Tits can usually be seen in large herds in gyms and locker rooms after New Years Eve day when people race to the gym to work off those extra pounds, but usually stick around until the summer time when they hibernate.
Steve: "Gross dude, look at that girl over there!"
Max: "Thanks for making me look man, my eyes are now burning after seeing those Basement Tits."
Patrick: "I think I should start wearing my girlfriends bra, I'm getting a set of Basement Tits from living off of Cheetos and Cupcakes while I play COD."
Jake: "I was so drunk that I thought I was getting lucky with two chicks at once, but she psyched me out with her Basement Tits."