Is it ensure, insure, or assure?
mid-15c., faulty separation (cf. adder, umpire) of a napron (c.1300), from Old French naperon "small table-cloth," diminutive of nappe "cloth," from Latin mappa "napkin." Napron was still in use as recently as late 16c. The shift of Latin -m- to -n- was a tendency in Old French (e.g. conter from computare, printemps from primum, natte "mat, matting," from matta). Symbolic of "wife's business" from 1610s. Apron-string tenure was in reference to property held in virtue of one's wife, or during her lifetime only.
Even at his age, he ought not to be always tied to his mother's apron string. [Anne Brontë, "The Tenant of Wildfell Hall," 1848]
found in the Authorized Version in Gen. 3:7, of the bands of fig-leaves made by our first parents. In Acts 19:12, it denotes the belt or half-girdle worn by artisans and servants round the waist for the purpose of preserving the clothing from injury. In marg. of Authorized Version, Ruth 3:15, correctly rendered instead of "vail." (R.V., "mantle.")