Cointreau

"Vintage Spirits & Forgotten Cocktails" by Ted Haigh (aka Dr. Cocktail):

"...before Prohibition the Cointreau bottle still read Cointreau Triple Sec. The liqueur was not only the first triple sec, it's how the term was coined. After imitators reproduced the signature square bottle and imprinted the words "Triple Sec" in the Cointreau typeface, Cointreau dropped the words from their bottle. The premium liquor has long since transcended the category, but that's how it started out...the first and best triple sec. ... Use generic triple sec only if you are short on cash."