impromptu
adj.done or said without any preparation or planning. You use this when someone does something on the spot because they have to or want to.
adj.planned, organized, or performed without advance preparation; extemporaneous. Often used to describe speeches, performances, or social gatherings.
“The team held an impromptu celebration after winning the game.”
“When the guest speaker failed to arrive, the professor gave an impromptu lecture on her recent research.”
“The jazz musicians engaged in an impromptu jam session that lasted until dawn, showcasing their ability to harmonize without a written score or prior rehearsal.”
Typically used attributively before a noun; can also follow linking verbs like 'be' or 'seem'.
“He takes me on an impromptu tour, showing me the living room that held Mouly’s huge, 1,000lb Multilith printing press, until they had a toddler and it became too dangerous to keep.”
“She looks mischievous when she thinks about the impromptu party, but declines to go into detail about what, exactly, went on.”
Borrowed from French impromptu.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.