Green’s Dictionary of Slang

1. an easy target, both in shooting and in metaphor; thus sitting adj., easy .

1900		Bulletin (Sydney) 1 Dec. 14/3: Many of these ‘crack shots’ were after the Governors, yet out of all shots fired at the outlaws only three bullets ‘landed,’ and those most probably by chance; although, according to reports, the pursuers had many ‘sitting’ shots at 100 yards, or less, with everything in their favour.
1923		Wodehouse Inimitable Jeeves 129: It’s a sitter for old Heppenstall.
1946		G. Gibson Enemy Coast Ahead (1955) 51: We must have been sitters.
1954		(con. 1940s) T.A.G. Hungerford Sowers of the Wind 158: Even when they’re sick in hospital, they reckon that if they can get you feeling sorry for them you’re a sitter.
1992		S. Armitage ‘Eighteen Plays on Golfing as a Watchword’ in Kid 83: A sitter fluffed from two feet.

2. a certainty.

1913

A. Lunn Harrovians 36: He was supposed to be ‘a sitter for his fez’.

3. a racehorse that is bound to win.

1923		Wodehouse Inimitable Jeeves 44: An absolute sitter came unstitched in the second race.
1954		Wodehouse Jeeves and the Feudal Spirit 13: The thing was pretty generally recognised as a sitter for me, last year’s runner up.