Something is said not to wash with Jeff if it would not go down well, in particular of an excuse
which is inadequate.
It comes from the line in Withnail and I
(Withnail has been reading in the paper about Jeff Wode, a grotesque steroid taking athlete
Withnail: Imagine the size of his balls. Imagine getting into a fight with the fucker!
Marwood: Please, I don't feel good.
Withnail: That's what you'd say, but that wouldn't wash with Jeff. No, he'd like a bit of pleading. Add spice to it. In fact, he'd probably tell you what he was going to do before he did it. (Starts acting out the scene in his head) "I'm going to pull your head off". "Oh no, please, don't pull my head off". "I'm going to pull your head off because I don't like your head."