The context and Latin original seem to suggest that the word is used in Dial Gregsatf with the meaning ‘face down, prostrate’, deviating from the etymological sense of 'face up, lying on one’s back' (cf. sovin).
a.
1face down, prostrate (?):
(
1212; MS: 1212-13
) E soi mist a la terre nue, supin en croiz molt umblement, E si orot [...] (Latin: sese in orationem dedit, in terram faciem depressit)10601
supin (1212)