Green’s Dictionary of Slang

Sir Timothy (Treat-all) n.

a very generous man.

c.1698		B.E. Dict. Canting Crew n.p.: Sir Timothy, one that Treats every Body, and Pays the Reckonings every where. [Ibid.] Squire of Alsatia [...] a Sir Timothy Treat-all.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue n.p.: Sir timothy. One who, from a desire of being the head of the company, pays the reckoning, or, as the term is, stands squire.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.