Is it ensure, insure, or assure?
early 15c., "increase efficacy" (of medicine); mid-15c., "provide (a town) with walls and defenses," from Old French fortifiier (14c.) "to fortify, strengthen," from Late Latin fortificare "to strengthen, make strong," from Latin fortis "strong" (see fort) + facere "to make" (see factitious).
Sense of "to strengthen mentally or morally" is from late 15c. Meaning "add liquor or alcohol" is from 1880. Related: Fortified; fortifying.