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to underestimate the size and/or weight of ones breasts.
be careful not to underbreastimate a women's tits; they are known to suffocate small animals and children!
by Matt Huff October 06, 2011
the act of underestimating the size of your boobs when trying on clothes
Girl: "I saw this really cute shirt and it looked like it would fit perfectly, but my boobs were practically popping out. I underbreastimated big-time"
by thatsdebateable2014 June 07, 2011