Green’s Dictionary of Slang

plumb v.¹

to fool, to deceive.

1804		Sporting Mag. May XXIV 137/1: Can you cog a die [...] Did you ever plumb the bones?
1878		G. Leybourne ‘Covent Garden’ in Comic Songs 12: To save all strife, take home to my wife / Some fruit and plum her a story.
1906		E. Pugh Spoilers 74: Kid wouldn’t go on like this if you ’adn’t never plumbed him up wi’ rotten lies about me.
1938		(con. late 18C) H. Asbury Sucker’s Progress 42: Loaded dice were called ‘dispatches’ and ‘dispatchers’ then as now, and to prepare them thus for cheating was to ‘plumb the bones’ or ‘load the doctors’.

In exclamations

go plumb! (excl.)

don’t try to fool me!

1902

L.R. Macleod ‘Consolation‘ in Bulletin Reciter n.p.: ‘Poor Casey’s gone on tramp.’ ‘Och, go plumb!’ said Mary, scolding.