The story
Apparently from Marco Polo's description of the legendary roc as being capable of seizing a small elephant.
Etymology from Wiktionary, CC BY-SA 4.0.
elephant bird
Today it means: Any of the very large extinct flightless birds that make up the genera Aepyornis and Mullerornis within family Aepyornithidae, and lived in Madagascar until the 17th or 18th century; especially †Aepyornis maximus. Full definition →