Definition: an object, such as a sponge, accidentally left inside a patient, following a surgical procedure.
Gossypiboma is a serious medical word for a serious thing, and should not be used metaphorically to refer to that thing that you forgot to grab when you left the house, and really (really) wish that you hadn’t left behind. But it could be used that way, especially if you are the sort of person who thinks other people’s suffering is occasionally amusing[1].
Source:
-1- https://www.merriam-webster.com/words-at-play/medical-words-you-should-know/gossypiboma