Green’s Dictionary of Slang

1. a bill; esp. in phr. meet sweet William, to pay off a bill as soon as it is presented; cit. 1929 suggests a play on the idea of the proper name ‘William’ being a ‘big Bill’.

1859		H.J. Byron Maid and Magpie 18: When de farmers around are behind in their rent I does little Villiams, at sixty per shent. He! he! hel he!
1864, 1867, 1870		Hotten Sl. Dict.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1895		Crowe Aus. Sl. Dict. 96: William, a bill.
1929		J. Lait Broadway Melody 63–4: When a bill gets into four figures it’s more than a bill — it’s a William.

2. (US) a dollar bill.

1865		Republican Banner 5 Oct. 3/1: Will. had to remember the Workhouse in his will to the tune of a ‘ten dollar William’ [DA].
1887		Lantern (N.O.) 21 May 5: The poor printer lost his five dollar William [...] and feels sore of it yet.
1909		L.W. Payne Jr ‘Word-List From East Alabama’ in DN III:v 388: william, n. A bank-note, a bill. ‘I’d give a ten-dollar william to see that.’.
1912		Amer. Mag. 306/1: Do I like New York, [...] Why, honey, I would give a five dollar William for just one breath from that dear desert, [...] or a neigh from old Black Eye [DA].
1927		C.A. Siringo Riata and Spurs 10: Mr. Myers wrote me [...] to buy a suit of clothes with the twenty-dollar ‘william.’.