Selena Gomez — Rings In 21st B-Day with GIANT BREAST CAN-SMASHING!!!

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Selena Gomez is having the BREAST birthday week ever — and kicked off the celebration by watching a big-hootered lady smash beer cans with her GINORMOUS BOOBS!!!!

Selena — who turned 21 yesterday — pre-partied at Beacher”s Madhouse in L.A. on Saturday night … where the singer and her crew had VIP seats to watch Busty Heart — world famous can annihilator.

We”re told Selena was sans Justin Bieber for the aluminum massacre … but he did make an appearance — in frosting.

Sources tell us … pictures of Justin and Selena were pasted onto a birthday cake that was decorated in leopard spots … a dessert clearly inspired by JB”s leopard-spotted Audi.

On her way out of the show, Selena appeared sober — which is good, considering she was still 20-years-old at the time.

Now, she”s fully legal … so, cheers!