off, n.¹ — Green’s Dictionary of Slang

off n.¹

[sporting jargon off, the start of any race, esp. that of horses or dogs]

the start.

1956		I. Fleming Diamonds Are Forever (1958) 40: So here it was. The ‘off’.
1959		F. Norman in Lilliput 1 Feb. in Norman’s London (1969) 75: I think I had better mention at the off [...] that I am not a tortured intellectual or anything like that.
1962		R. Cook Crust on its Uppers 96: I don’t suppose I shall see you again before the off.
1977		G.F. Newman A Prisoner’s Tale 19: You’d better understand something right from the off.
1999		Indep. Rev. 19 July 1: I rather like Fatboy Slim from the off.
2000		N. Barlay Crumple Zone 119: A suden rush of anger, fizz of adrenalin. Like I’m all set for the off.