1540s, "words, clauses, etc. inserted into a sentence," from Middle French parenthèse (15c.), from Late Latin parenthesis "addition of a letter to a syllable in a word," from Greek parenthesis, literally "a putting in beside," from parentithenai "put in beside," from para- "beside" (see para- (1)) + en- "in" + tithenai "to put, to place," from reduplicated form of PIE root *dhe- "to set, put." Sense extension by 1715 from the inserted words to the curved brackets that indicate the words inserted.
A wooden parenthesis; the pillory. An iron parenthesis; a prison. ["Dictionary of Buckish Slang, University Wit and Pickpocket Eloquence," London, 1811]