Green’s Dictionary of Slang

to raise one’s hat to an acquaintance.

1812		Vaux Vocab. of the Flash Lang. in McLachlan (1964) 247: kelp to kelp a person, is to move your hat to him.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1843		Satirist & Sporting Chron. (Sydney) Feb. 4/3: I wears all the rim off my hat [...] kelping men as calls themselves noblemen.