early 13c., "personally known;" past-participle adjective from acquaint (v.). Of skills, situations, etc., from late 15c.
I have been one acquainted with the night.
I have walked out in rain — and back in rain.
I have outwalked the furthest city light.
[Robert Frost, from "Acquainted with the Night"]
Acquaint also was used as an adjective (late 13c.) "acquainted."