Green’s Dictionary of Slang

my hat! excl.

a general excl.

1897		Kipling ‘Slaves of the Lamp — Part I’ in Complete Stalky & Co. (1987) 58: ‘My hat! said he, throwing himself upon the banquet. ‘Who stumped up for this, Stalky?’.
1905		Sporting Times 1 Apr. 11/2: Said No. 2: ‘My hat! this is a really nice girl.’ Said No. 1: ‘She is a nice girl, old chap, but that was my foot.’.
1919		‘Sapper’ Mufti 268: My hat! [...] Can this thing be true? Giving up leave.
1922		E. Raymond Tell England (1965) 117: ‘My hat!’ exclaimed White.
1934		E. Raymond Child of Norman’s End (1967) 69: Sunday School kids! [...] My hat!
1935		G. Blake Shipbuilders (1954) 163: ‘My hat, Danny!’ he cried.
1945		J.B. Priestly Three Men in New Suits 75: My hat, they would, would they?
1961		‘Frank Richards’ Billy Bunter at Butlins 40: Oh, my hat!
1977		‘John le Carré’ Honourable Schoolboy 254: My hat! F-ing and blinding like I don’t know what.
1986		R.G. Barrett Godson 119: ‘[M]y hat. Some of them look old enough to be your aunty’.
1989		R. Dahl Rhyme Stew (1990) 39: A brazen wench cried, ‘Oh my hat!’.
1996		D. Healy Bend for Home 86: We’re on holidays, said Matt Donnelly. You are in my hat. Do you think I’m a gom?
2017		Times 22 May 🌐 He did not swear. ‘Gracious!’ and ‘my hat!’ did duty as his most violent expletives.

In exclamations

my hat to a halfpenny!

a general excl.

c.1595

Shakespeare Love’s Labour’s Lost V ii: My hat to a halfpenny, Pompey proves the best Worthy.