impromptu - Dictionary definition and meaning for word impromptu
just remember prompt of in[promptu] that's it
im(in-without)-prompt-u i.e. without prompting, u answer which is extemporaneous or spontaneous.
when we tell some lie at home and suddenly our friend comes their, we do not need to prompt him about telling lie and handling the situation. He does not have to be told about it or has to come prepared for it. Friends can handle.
U "Prompt" "Im"mediately without preparation