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late 14c., "shaving of the head or part of it as a religious rite," from Anglo-French tonsure (mid-14c.), from Old French tonsure (14c.), from Latin tonsura "a shearing, clipping," from tonsus, past participle of tondere "to shear, shave," from PIE *tend-, from root *tem- "to cut" (see tome). The verb is attested from 1793. Related: Tonsured; tonsuring.
in various religions, a ceremony of initiation in which hair is clipped from the head as part of the ritual marking one's entrance into a new stage of religious development or activity