Green’s Dictionary of Slang

1. common sense; esp. in adjuration have/use a bit of common, use your common sense.

1906		E. Dyson Fact’ry ’Ands 8: Jimmy Jee! She’s no fact’ry rat. Do ’ave er bit iv common.
1911		L. Stone Jonah 101: ’Ave a bit o’ common, an’ don’t make a ’oly show of yerself ’cause yer lost a dollar.
1936		J. Curtis Gilt Kid 175: You’ll get us done, yelling around the gaff like that. Use a bit of common.
1943		R. Llewellyn None But the Lonely Heart 184: We can’t kip in a bleeding wash stand, can we? Use a bit of common.
1960		H. Pinter Dumb Waiter 16: You mutt... Have a bit of common. They got departments for everything.

2. common decency.

1910

Bulletin (Sydney) 1 Dec. 18/1: Shut up, you blokes! Bert’s on a good thing now! / Gorstrooth, you are a lot of flamin’ cows! / Do ’ave a bit o’ common! Shut yer row!