The Collaborative International Dictionary of English v.0.48:
Parenthetically \Par`en*thet"ic*al*ly\, adv. In a parenthetical manner; by way of parenthesis; by parentheses. [1913 Webster]WordNet (r) 3.0 (2006):
parenthetically adv 1: in a parenthetical manner; "he added parenthetically that he would not attend the wedding ceremony"