Unpremeditated (adjective)

1. Not planned or considered beforehand.
2. Not having been given prior thought or consideration.
3. Spontaneous and unplanned.

Origin:

Late 16th century, from un- + premeditated.

Examples:

1. The unpremeditated remark caused a heated debate among the audience.
2. The unpremeditated act of kindness brought tears to her eyes.
3. The unpremeditated trip was the best one they had ever taken.
4. The unpremeditated concert was a great success.
5. The unpremeditated visit was a pleasant surprise.