nSept-foilsept′-foil a plant, the roots of which are used in medicine, tanning, &c.: a figure of seven equal segments of a circle used in the R.C. Church as a symbol of her seven sacraments, the seven gifts of the Holy Spirit, &c.
Chambers's Twentieth Century Dictionary
Fr. sept—L. septem, seven, foil—L. folium, a leaf.
This is beneath a sept-foiled arch, beside which is another strangely irregular arch over a sedile.
"Highways and Byways in Cambridge and Ely" by Edward Conybeare