skewback

plural skewbacks

1. architectureThat portion of an abutment which is arranged to receive the force of a segmental or flat arch, having an inclined face corresponding with the adjoining voussoir. The side of an arch; the course of masonry on the top of an abutment with a slope for the base of the arch to rest against.

   ...fit it upon the leading skewback, or leading end of the arch.

   1860, Rudimentary Dictionary of Terms Used in Architecture, Civil, Architecture Naval, Building and Construction... etc., J. Weale, page 48

   2 more examples

   That portion of an abutment which is arranged to receive the thrust of a segmental or flat arch, having an inclined face corresponding with the adjoining voussoir.

   1902, Russell Sturgis, A Dictionary of Architecture and Building, Vol. 3, Gale Research Co., page 523

   The avalanche shelter itself consists of a series of heavily reinforced arches, supported by substantial piers on the cliff-side, and on skewbacks cut deep into the rock on the other.

   1943 March and April, “A British Avalanche Shelter”, in Railway Magazine, page 80