Exact historical dictionary sense
Prior in Webster's 1913 dictionary
The 1913 record labels prior as adjective. Its source-bounded etymology and selected historical definition appear below.
[L. prior former, previous, better, superior; compar. corresponding to primus first, and pro for. See Former, and cf. Prime, a., and Pre-, Pro-.]
Cross-verified historical senseIn Webster's Revised Unabridged Dictionary (1913), "prior" is labeled adjective. Its etymology is recorded as: "[L. prior former, previous, better, superior; compar. corresponding to primus first, and pro for. See Former, and cf. Prime, a., and Pre-, Pro-.]" The selected historical definition says: "Preceding in the order of time; former; antecedent; anterior; previous; as, a prior discovery; prior obligation; -- used elliptically in cases like the following: he lived alone [in the time] prior to his marriage."