I just heard this word for the first time, from a California criminal defense lawyer I know; it seems to be a purely California legalism — the over 30 uses I found on Westlaw, starting with the mid-1980s but accelerating in the last ten years, all come from California.
Many sentencing schemes dramatically enhance a person’s sentence if he has been convicted in the past of certain kinds of prior offenses. A “priorable” offense is an offense that can count as a “prior” under such a scheme; the term is usually with some particular scheme, which is clear from context, in mind. There is also apparently an older meaning in botany, which refers to a term’s having priority for purposes of naming a newly identified species.
I’m not praising this term, or urging that it be used more broadly; but I thought it was worth noting.