dissociation n.

1610s (implied in dissociated) "sever the association or connection of," especially "cut off from society," from Latin dissociatus, past participle of dissociare "to separate from companionship, disunite, set at variance," from dis- "apart" (see dis-) + sociare "to join," from socius "companion, ally," from PIE *sokw-yo-, suffixed form of root *sekw- (1) "to follow."

Attested from 1540s as a past-participle adjective meaning "separated." Dissociated in psychology (1890) was "characterized by mental disjunction," hence dissociated personality (1905) "pathological state in which two or more distinct personalities exist in the same person."