This verb, meaning "to improvise," "to deliver spontaneously," is derived from Latin ad libitum, meaning "at pleasure." It can correctly be used to mean "freely" but is often overused. Ad-lib is a trite expression when used to mean adding words or gestures hastily improvised, or concocted, that are not in the script or are not intended to be expressed by word or action.
Dictionary of problem words and expressions. Harry Shaw. 1975.