From Longman Dictionary of Contemporary Englishad-libad-lib /ˌæd ˈlɪb/ verb (ad-libbed, ad-libbing) [intransitive, transitive]PERFORM to say things that you have not prepared or planned when you are performing or giving a speech SYN improvise I never use a script; I just ad lib the whole programme. —ad-lib noun [countable]→ See Verb table
Examples from the Corpus
ad-lib• No one could remember the song very well, so we had to ad-lib.• The other actors were thrown into confusion when she started ad-libbing her final speech.• Betsy forgot her lines and had to ad-lib the rest of the scene.Origin ad-lib (1900-2000) ad lib “without any preparation” ((19-21 centuries)), from Modern Latin ad libitum “according to desire” 