off-the-cuff

Not prepared in advance; impromptu: an off-the-cuff remark.

adjective

Spoken, performed, or composed with little or no preparation or forethought: ad-lib, extemporaneous, extemporary, extempore, impromptu, improvised, offhand, snap, spur-of-the-moment, unrehearsed. See prepared/unprepared.

Impromptu, extemporaneous, as in His speech was entirely off the cuff. This term supposedly alludes to the practice of speakers making last-minute notes on the cuff of a shirtsleeve. [1930s]

The adjective has one meaning:

Meaning #1: with little or no preparation or forethought
  Synonyms: ad-lib, extemporaneous, extemporary, extempore, offhand, offhanded, unrehearsed