Green’s Dictionary of Slang

considerable in amount.

1861		(con. 1840s–50s) H. Mayhew London Labour and London Poor I 291/1: I sell Lloyd’s and Reynold’s pennies – fairish, both of them.
1890		‘Rolf Boldrewood’ Colonial Reformer I 74: You remember me a fairish time, Joe.
1891		H.B. Marriott-Watson Web of the Spider 235: A fairish heap of gold, collected over many years.