leper's squint

low-side window, leper’s squint, offertory window, squint

A small low window, usually on the right side of the chancel, through which the altar may be seen.
References in periodicals archive ?
The church itself is of sandstone, mainly dating from 1860 but on the site of its medieval predecessor, still mentioned for its surviving leper's squint in a 14th century north wall.
Hence the ancient name for such an opening, "the leper's squint.