ad lib / ˈad ˈlib/ • v. (ad libbed, ad lib·bing) [intr.] speak or perform in public without previously preparing one's words: Charles had to ad lib because he'd forgotten his script | [tr.] she ad libbed half the speech. • n. something spoken or performed in such a way.• adv. & adj. 1. spoken or performed without previous preparation: an ad lib commentary | [as adv.] speaking ad lib. 2. as much and as often as desired: [as adv.] the price includes meals and drinks ad lib | [as adj.] the pigs are fed on an ad lib system.