extempore: [16] An extempore speech is one that is given literally ‘out of time’ – that is, ‘on the spur of the moment’. That was the meaning of the Latin phrase extempore (ex ‘out of’ and tempore, the ablative case of tempus ‘time’), which was the source of the Latin adjectives extemporālis and extemporāneus. Both of these were acquired by English, as extemporal [16] and extemporaneous [17], but only the latter has survived. Ex tempore itself was first lexicalized in English as an adverb, and was not used as an adjective until the 17th century.
extempore (adv.)
1550s, from Latin phrase ex tempore "offhand, in accordance with (the needs of) the moment," literally "out of time," from ex "out of" (see ex-) + tempore, ablative of tempus (genitive temporis) "time" (see temporal). Of speaking, strictly "without preparation, without time to prepare," but now often with a sense merely of "without notes or a teleprompter." As an adjective and noun from 1630s.