An onomatopoeic "in your face." Originally the sound of a slam-dunk in basketball -- the "boom" being the dunk, and the "shakalaka" being the rattling of the backboard.
Pippen over to Grant ... he shakes and bakes ... to the basket ... BOOMSHAKALAKA! (NBA Jam announcer)
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).