also protonotary (under which spelling it appears in OED print edition), mid-15c., "principal clerk of a court," from Medieval Latin prothonotarius, from a Late Latin borrowing of Greek prōtonotarios "first scribe," originally the recorder of the court of the Byzantine empire, from prōtos "first" (see proto-) + Latin notarius (see notary). Related: Prothonotarial.
The -h- appeared in Medieval Latin, perhaps because Greek prōto- sometimes became prōth- (before an aspirated vowel); it was carried into Old French, which passed it to Middle English. Other Middle English proto- words from French also had variants in protho- (prothomartir "earliest martyr," Protheus "the god Proteus," prothogol "protocol," all 15c.), but soon it was purged from the others; prothonotary kept its perhaps through the powerful and necessary conservatism of legal language.