ad-lib

Concise Oxford English Dictionary © 2008 Oxford University Press:
ad-lib/adˈlɪb/

▶verb (ad-libs, ad-libbing, ad-libbed) speak or perform in public without preparing in advance. ▶adverb & adjective

• 1 spoken without advance preparation.

• 2 Music with free rhythm and expression.

• 3 as much and as often as desired.

▶noun an ad-lib remark or speech.

– origin C19: abbrev. of ad libitum.