It's a Dilbertism,
isn't it?
( DogHorse wants to do a poo in Paul's bathroom, Tue 3 Aug 2004, 15:57, archived)

I believe
so
( MakTheYak, Tue 3 Aug 2004, 16:01, archived)

It predates Dilbert.
It probably originated on talk.bizarre, an old Usenet newsgroup.
( Newington HM Time Corps: Quo vadimus non requirimus vias, Tue 3 Aug 2004, 16:13, archived)