ad lib

also ad lib., 1811 as a musical instruction, shortened from Latin ad libitum "to (one's) pleasure, as much as one likes" (c. 1600), from ad "to" (see ad-) + libitum "pleasure," accusative of libere "to please" (see libido). As a noun from 1825; as a verb by 1915.

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Definitions of ad lib from WordNet

ad lib (adv.)
without advance preparation;
he spoke ad lib
Synonyms: ad libitum / spontaneously / impromptu