Green’s Dictionary of Slang

a general excl. of emphasis; thus ext. as …sacred Sam!, …sainted Sam!

1879		‘Somerset Frank’ (F. J. Squires) Nine Days in Devon 12: Or ’pon my zam! oi really think as Zal’d a zot on moine [OED].
1891		Manchester Courier 12 Sept. 13/4: I declare you are though; pon my sacred Sam, you are, you’re gettuing quite a bow-window.
1898		Kipling ‘Stalky’ in Complete Stalky & Co. (1987) 24: ’Pon my Sam, you Burrows guv’nors haven’t any sense. [Ibid.] 26: ’Pon my sainted Sam, that’s awfully good of you. [Ibid.] 40: ’Pon my sacred Sam, though, it’s enough to drive a man to drink.
1900		Marvel 15 May 14: ’Pon my Sam, it’ll be worth yer money ter see us!
1908		Western Times 4 Dec. 14/4: ‘Observer’ said ‘Upon my Sam, This quite beats Washford Pyne’.
1920		Marvel 9 Oct. 15: Upon my Sam, did you ever see anything like it in your natural life?
1926		Times Colonist (Victoria, BC) 17 July 28/7: He brought his fist down with a crash upon the table ‘Upon my Sam!’.
1935		Eve. Teleg. (Dundee) 21 Feb. 9/5: ‘Pon my sam, sir [...] You were right after all.
1943		R. Llewellyn None But the Lonely Heart 116: ‘Upon my blinking sam,’ says Marjoribanks.