Triple sec, originally Curaçao triple sec, is a type of strong, sweet and colorless orange-flavored liqueur. It is a variety of Curaçao liqueur, an orange-flavored liqueur made from the dried peels of bitter and sweet oranges.
The etymology of "triple sec" is unclear. "Sec" is the French word for dry. Triple sec may refer to the schnapps being distilled three times, having triple the flavor, being three times as dry as other spirits, or it could just be a marketing gimmick.
The Combier distillery claims that triple sec was invented in 1834 by Jean-Baptiste Combier in Saumur, France. However, the Combier distillery was more famous for its élixir Combier, which contained orange but also many other flavorings.
According to Cointreau, its orange liqueur was created in 1875.
Triple sec was certainly widely known by 1878; at the Exposition Universelle of 1878 in Paris, several distillers were offering "Curaço [sic] triple sec", as well as "Curaço doux".
^"Les liquoristes saumurois" in Saumur jadis, anonymous Web publication at  cites Richard Gasnier, Les liquoristes saumurois de 1830 à 1910, mém. de maîtrise, Angers, 2000, B. U. de l'U.C.O., 15 747 ; François Bouyssi et Isabelle Emeriau, "James Combier (1842–1917 ). Essai biographique...", S.L.S.A.S.,, 1992, pp. 46–89 ; Alain Mariez, "Un zeste d'orange, deux doigts d'ambition", L'Anjou, décembre 1995, pp. 70–77 ; Christelle Couvreux, Marie Bardisa, La Distillerie Combier. Saumur, Itinéraires du Patrimoine, 1999.
^The Lancet Analytical Commission, "Report on the Food Products exhibited in the French and English Departments of the Universal Exhibition of Paris", The Lancet, September 21, 1878, p. 417f.