Something that is made up on the spot and given little time to gather and present. Usually referring to speeches that are given only a few minutes to prepare for.
I had to write an impromptu speech about Anal Cancer in 3 minutes without using the word 'ass' 'anus' 'shit' or 'hair'.
On the spot
When my chik walked in on me with my mistress, I had to make up an impromptu excuse for why my purple headed yogurt slinger was in her tuna taco.
Something that is made up on the spot. Can also mean a speech that was made with little or no preparation.
The drug dealer had to come up with an impromptu excuse to his boss for losing the shipment of crack to the police.