tilde (n.)
1864, from Spanish, metathesis of Catalan title, from vernacular form of Medieval Latin titulus "stroke over an abridged word to indicate missing letters," a specialized sense of Latin titulus, literally "inscription, heading" (see title (n.)). The mark itself represents an -n- and was used in Medieval Latin manuscripts in an abridged word over a preceding letter to indicate a missing -n- and save space.