The awkward amount of breast protruding from beneath a bikini top. It is debatably sexy, but it is def a spring break must.
Wow, That girl on sweet 16 has the under boob thing going on..
Poor Nicole Ritchie, she doesn't have enough body fat to pull off the under boob.
The under boob is very ellusive and rarely shows itself. To coax, said the under boob from hiding, one must adjust thine own bra to a point of maximum comfort. The constriction of the bra causes under boob to show underneath the restricted area pleasing those around, that in which interns to show gratitude shall rejoice in joyfull song and dance;giving it the title, "UNDER BOOB."
Tim: "Yo Sam, tuck that Under Boob away, you're giving me a boner!"
Sam: "Oh man, I apologize Tim, I constricted my bra to a maximum, and it just well, popped out!"