This rarely attested word (there is one other instance in Middle English, half a century later) adds a superfluous adjectival -ous suffix. The process is similar process to that of pensantus.
a.
1pensive, preoccupied:
(
c.1334; MS: s.xivm
)(Gereberd is refused access to a certain mysterious book:) mult pensifs (var. (T: s.xiv3/4) pensivous) estoit, et tant come cel livre lui estoit denié, taunt plus le desira
277.23
pensivous (s.xiv3/4)