Green’s Dictionary of Slang

a punch in the face; usu. in phr. give someone a doughboy, to punch someone in the face.

1980		Barltrop & Wolveridge Muvver Tongue 69: ‘I didn’t half pay him’ [...] is another way of saying that he received a domino, or a right doughboy.
1986		J. Sullivan ‘Tea for Three’ Only Fools and Horses [TV script] I’m gonna land him a doughboy right round the lug ’ole before he’s much older!