Newmarket pound, n. — Green’s Dictionary of Slang

Newmarket pound n.

[the use of guineas (£1.05p) rather than pounds for transactions at Newmarket, a centre of horseracing and breeding]

a guinea, one pound and one shilling.

1775

Proceedings Old Bailey 31 May 323/1: She (Mrs. Rudd) said, it will cost three thousand Newmarket pounds; upon which Mrs. James Adair asked, what is Newmarket pounds? and Mrs. Rudd said, guineas: upon which Mrs. James Adair said, she thought pounds might serve very well, and she would not give any more.