The word can be considered an early attestation of 'failure', a word that only appeared in English after the Middle Ages (17th century, see OED). The word can alternatively be interpreted as an unusual variant of
folur.
s.
venir a faillour
1to fail, come to an end:
(
s.xiii1; MS: s.xiiim
)
Mult fud bone la vile, onc home ne vit meillur [...] Mais au tens que la pristrent nostre bon ancesur Esteit ja destruite et venue a foillur (var.
(S: c.1400) faillour
)3442
faillour (s.xiii1)