late 14c., sessioun, "periodical sitting of a court," from Old French session "act or state of sitting; assembly," from Latin sessionem (nominative sessio) "act of sitting; a seat; loitering; a session," noun of action from past-participle stem of sedere "to sit" (from PIE root *sed- (1) "to sit").
By 1550s in the general sense of "the time or term during which a legislature, etc. meets daily for business." The still more general sense of "period set aside for some activity" is recorded by 1920, in bull session, which probably is extended from quarter sessions courts (see quarter (n.1)).
The musical sense of "recording occasion in a studio" is from 1927. The literal classical sense has been rare in English except in theology, of Christ's enthronement at the right hand of the Father (early 15c.). Related: Sessional.