A Florida man was taken into police custody last week … and cops made a most unexpected discovery jammed all the way up his butt — a full-size thermos! — but that’s only half the story.

TMZ has obtained exclusive 911 audio from the sh—y situation in which a caller reports there’s a naked man in a restroom at a public park who had “been there all day and won’t leave.”

TMZ has learned sheriff’s deputies showed up and found him fully clothed … they initially gave the guy — identified as Walter Frymire — a break, escorting him out of the Winter Haven park with just a warning … but later, Frymire allegedly trespassed again.

Instead, there on the screen was a thermos shoved so far up Frymire’s rectum, it had made it into his abdomen.

Cops say Walter was a little testy even before making it to the processing center, allegedly telling them … “Good, I’m going to kill you, so charge it. If you arrest me, I will find you and kill you.”

Guess now we know why he was in such a bad mood! Either the bottle up the bum, or it could’ve been the “glass pipe with a burned residue consistent with methamphetamine use” authorities say they found inside a gray lunch box Walter was carrying that was making the 51-year-old cranky.

After finding the fanny flask, the Polk County Sheriff said they had Frymire transported to a hospital where the thermos was removed, likely saving his life.