cap acquaintance, n. — Green’s Dictionary of Slang

cap acquaintance n.

[a person one knows well enough to raise one’s cap when one passes on the street, but not to speak to properly]

a slight or passing acquaintance.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Cap-acquaintance. Persons slightly acquainted, or only so far as mutually to salute with the Hat on meeting.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn).
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.