The FEW has not identified the etymon for the term, while the OED suggests a derivation from late Latin circellus 'small circle'.
s.
1zool.orn.pinnion, primary feather of bird's wing:
(
MS: 1272-82
)
Li saiseus (ms. saisefius)(var.
(A: s.xiv1/3) seaus
) ceo est la premerraine penne des dreites pennes des eles. Li cutels, ces sunt les .vj. plus longes pennes des eles aprés le cessel (var.
(A: s.xiv1/3) seel
)95.13
[saisel] (1272-82)