Diarrhea. "Backdoor" refers to the now-outdated practice of using an outhouse. Something I ate has given me a serious case of the backdoor trots, ugh.
See also: backdoor
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
n. a case of diarrhea. (From the time when people had to go out the back door to the outhouse.) I can’t go out tonight. I got a case of the backdoor trots.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.