Pea wet is the waste product left behind by mushy peas.
Whereas Mushy Peas are mushy n thick pea wet is a poor mans equivalent and is the water thats left behind after the peas are made.
It looks the urine of somebody with a bladder or genital infection but tastes good with chips.
(or so I'm told)
It is also used as an expression for getting Finger Pie from a dubious lady, usually whilst in a drunken stupor