early 13c., from Old French dragon, from Latin draconem (nominative draco) "huge serpent, dragon," from Greek drakon (genitive drakontos) "serpent, giant seafish," apparently from drak-, strong aorist stem of derkesthai "to see clearly," from PIE *derk- "to see." Perhaps the literal sense is "the one with the (deadly) glance."
The young are dragonets (14c.). Obsolete drake "dragon" is an older borrowing of the same word. Used in the Bible to translate Hebrew tannin "a great sea-monster," and tan, a desert mammal now believed to be the jackal.