go down the toilet
Verb
Meaning
(idiomatic, figuratively) To fail.
(idiomatic, figuratively) To degenerate rapidly.
Used other than figuratively or idiomatically: see go, down, the, toilet.
Source: en.wiktionary.orgSynonyms
fail
slide
deteriorate
worsen
retrogress
go to the dogs
go to hell
go down the khazi
go to seed
go down the pan
go down the tubes
go down the drain
go down the tube
go down the plughole