*sekw- (1)

mid-15c., "join in company, combine intimately" (transitive), from Latin associatus past participle of associare "join with," from assimilated form of ad "to" (see ad-) + sociare "unite with," from socius "companion, ally" (from suffixed form of PIE root *sekw- (1) "to follow"). Related: Associated; associating.

The intransitive sense of "have intercourse, be associated" is from 1640s. The earlier form of the verb was associen (late 14c.), from Old French associier "associate (with)."

1530s, "action of coming together for a common purpose," from Medieval Latin associationem (nominative associatio), noun of action from past-participle stem of associare "join with," from assimilated form of ad "to" (see ad-) + sociare "unite with," from socius "companion, ally" (from PIE *sokw-yo-, suffixed form of root *sekw- (1) "to follow").

The meaning "an organized body of persons with a common purpose" is from 1650s. The sense of "mental connection" is from 1680s; that of "quality or thing called to mind by something else" is from 1810.