Green’s Dictionary of Slang

a mild excl. of surprise or amazement.

c.1595		Shakespeare Love’s Labour’s Lost V ii: To begin, wench, – so God helpe me, la! – My love to thee is sound sans crack or flaw.
1678		Otway Friendship in Fashion V i: Oh law! my Aunt! what have I done now?
1681		Otway Soldier’s Fortune I i: Bubbies! oh law, there’s bubbies! – odd, I’ll bite ’em; odd, I will!
1697		Cibber Woman’s Wit III v: O law! did I never tell you how she serv’d an amorous Book of Major Rakish’s t’other day.
1700		T. Brown Amusements Serious and Comical in Works (1744) III 25: Indeed, lau, no, saith the merchant.
1773		Foote Bankrupt III i: O law! You are in a vast prodigious great hurry.
1821		W.T. Moncrieff Tom and Jerry III iii: Law, lovee, no, it’s only some gemmen out on the spree.
1837		Thackeray Yellowplush Papers Works III (1898) 244: ‘Law, miss,’ said I, ‘what shall I do?’.
1841		D. Boucicault London Assurance Act II: pert: The dear old man! Do you mean Sir Harcourt? grace: Law, no! My uncle, of course.
1859		E. Eden Semi-Detached House (1979) 149: Law, Sir! that is ease! that is ease!
1871		B. Harte ‘Penelope’ in Poems 77: Law! why, sho! I’m as weak as a gal.
1887		W.S. Gilbert ‘The Precocious Baby’ Fifty ‘Bab’ Ballads 62: With their ‘Law, dear me!’ / ‘Did you ever see?’.
1900		Boy’s Own Paper 10 Nov. 83: Law, Mr. Monslow: what have you been a-doin’ of?
1910		H.G. Wells Hist. of Mr Polly (1946) 184: ‘Law!’ she said [...] ‘I thought you was Jim.’.
1922		E. Raymond Tell England (1965) 41: ‘O law!’ I said.
1934		E. Raymond Child of Norman’s End (1967) 48: But law! your ma isn’t half in a stew.
1996		J. Cameron It Was An Accident 169: Dear oh law what you want, blood?