Keenan & Shawn Wayans -- Sued By Law Firm -- We're Gonna Get You Suckas!

Keenan Wayans and his little bro Shawn Wayans owe a bundle of cash to an L.A. law firm -- according to a new lawsuit -- but relax ... they shouldn't have to make a "White Chicks" sequel to pay it off.

According to court docs -- obtained by TMZ -- the Wayans brothers stiffed the firm for more than $63K in legal services they received back in 2008-09.

The firm -- S & E Azriliant -- claims Keenan owes $21,825 ... and Shawn owes $22K. Additionally, they owe another $20,200 together. Grand total -- $63,485, PLUS interest.

The really sad part -- around the time Keenan and Shawn ran up the legal bill, they made the movie "Dance Flick." Classic lose-lose.