dipstick, n. — Green’s Dictionary of Slang

dipstick n.

[play on SE; widely popularized by the BBC TV series ’Only Fools and Horses (from 1981), whose star, David Jason, refused to use obscenities, but dipstick fits neatly in the range of words that mean both penis and fool, e.g. dickhead n., dork n.; also popularized by the character ‘Boss Hogg’ in US TV show The Dukes of Hazzard; note 19C ‘a person [...] who ascertains the capacity or content of a cask or similar vessel using a gauging rod’ (OED) also used as a derog. nickname]

1. the penis.

1969		in Current Sl. IV:3–4 (1970) 20: Lipstick on a dipstick, n. Oral-genital sexual relations.
1980		Maledicta IV:2 (Winter) 191: Puns occur with the terms joy-stick and dipstick — I overheard in a cinema once the cry ‘Keep your lipstick off my dipstick!’.
2000		S. Maloney Big Ask 101: Half an hour later, i was pinned against the back window of the Kenilworth with Heather’s lipstick on my dipstick.

2. (orig. US) a fool, an incompetent.

1968		Current Sl. II:4 4: Dipstick, n. A dumb or objectionable person.
1977		L. Heinemann Close Quarters (1987) 149: Trobridge, that puffy-faced dipstick.
1981		J. Sullivan ‘Big Brother’ Only Fools and Horses [TV script] A lot of people told me I was a right dipstick to make my brother a partner in the business.
1993		T. Jones Pugilist at Rest 147: You want to know who she was fucking, dipstick? She was fucking Centrick Cline; need I say more?
2000		N. Griffiths Grits 450: Liam an Colm singing their stupid fuckin rebel songs [...] Pair av bleedin dipsticks.
2002		S. Maloney Something Fishy (2006) 83: We were almost totalled by a dipstick in an Audi.
2010		L. Berney Gutshot Straight [ebook] ‘The dipstick changed his locks’.
2016		N. Cummins Adventures of the Honey Badger [ebook] He’s dirty on everything. You couldn’t get a smile out of him at gunpoint. A real dipstick and a 10/10 prick.