General terms:
* Impulsive: Acting suddenly and without thinking.
* Spontaneous: Done without planning or being forced.
* Unpremeditated: Not planned in advance.
* Unprepared: Not ready for something.
* Extemporaneous: Done or spoken without preparation.
* Off-the-cuff: Done or said without prior thought or preparation.
More specific terms:
* Rash: Acting quickly and without thinking carefully, often with negative consequences.
* Impetuous: Acting quickly and with strong emotion.
* Careless: Not taking care or paying attention.
* Reckless: Showing a lack of care for danger or risk.
Figurative language:
* Off the top of one's head: Said or done without thinking carefully.
* On the fly: Done or made without prior planning.
The best word to use depends on the specific context and the nuance you want to convey.