A view of the female breast where only the areola/nipple is covered. This is usually performed with two or more fingers for each breast. Peekaboob is related to sideboob and underboob because both areas can be seen.
Too many photos in US mens magazines that can't actually show nudity so they resort to peekaboob.
Like peekaboo, only instead of removing your hands to reveal your face, you pull up/down the shirt to reveal the boobies, then you quickly hide them away again, as in the childhood classic.
NOTE: It is best to play without warning, right in the middle of her telling you something really important.
Anna: Honey, I really want you to know that I-
Anna: God Dammit you are such a typical guy!