The story
Initially, the TED spread was the difference between the interest rate for the three month U.S. Treasuries contract and three month Eurodollars contract as represented by the London Inter Bank Offered Rate. So "TED" came from the initialism of "T-Bill" and "ED"—the ticker symbol for the Eurodollar futures contract. Since the Chicago Mercantile Exchange dropped the T-bill futures, the TED spread has had the current definition.
Etymology from Wiktionary, CC BY-SA 4.0.