The phrase "wast a party to" is an archaic way of saying "you were part of it." The spelling of "wast" is based on the Middle English pronunciation of "was," which was pronounced as /wɑs/ or "wahs." The "t" at the end of "wast" indicates that this word is in the second person singular past tense. The phrase "a party to" is a legal term that means being involved or participating in something. Together, "wast a party to" means "you were involved in it."