Green’s Dictionary of Slang

dipstick adj.

(US) stupid.

1986		L. Heinemann Paco’s Story (1987) 4: And the geeks and freaks [...] hear the dipstick yokels soaking up a shill like that.
2000		S. Maloney Big Ask 98: Suddenly dipstick Darren was the great white hope.
2015		I. Welsh Decent Ride 116: This posh-doorman-type ay cunt [...] wearin a dipstick uniform.