"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"
OR
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!"