"Smooth move" is a sarcastic comment in response to a stupid or crass action and "exlax" is a pejorative reference to the perpetrator of such action. The whole term comes, of course, from the fact that Exlax enables smooth bowel movements.
The insulting capability of "smooth move exlax" runs the gamut from a mild put down (as to an friend who just said or did something reasonably stupid) to a scathing remark on the inane actions of a worthy idiot.
To a friend not watching were she is walking and who runs into a sign post: "Smooth move exlax"
To a submoron who realizes he was about to miss his freeway exit and proceeds to cut across three lanes of oncoming traffic, causing angry motorists to brake and nearly crash: "SMOOTH MOVE, EXLAX!"
When someone makes a not so smooth move, one would use this phrase in a verbally ironic fashion.
Exlax is a common laxative which causes "smooth movements" of the bowels so to speak.
By using this statement, you are comparing a persons action to that of a laxative.
"You guys, I left the door to the bathroom unlocked, and just got caught wanking off
"Smooth move exlax"