Is it ensure, insure, or assure?
: They danced ad lib until the conductor found his place againmodifier
: an ad-lib gag/ a quick ad-lib put-downnoun phrase
A passage or comment, etc, given spontaneously: He forgot his lines and did a stupid ad libverb phrase
To speak, play, dance, or otherwise perform a passage not in one's prescribed plan, often with an original and spontaneous effect: The Senator ad lib-bed for ten minutes waiting for the President to show
[fr Latin ad libitum, ''as one wishes'']