This word may in fact be a misread form of purpenser. Similar forms are attested in CF in the early fifteenth century while English (prepensed adj. is attested from 1454. It is attested in the sixteenth century in Latin (praepensare, DMLBS 2417c is unattested before 1562). This reformation of purpenser suggests that the prefix was reinterpreted (prae+ instead of pro+) to reflect the sense of anteriority present in the word.