"very large flightless bird inhabiting the sandy plains of Africa and Arabia," early 13c., also hostriche, estrich, ostrig, esterige, etc., from Old French ostruce "ostrich" (Modern French autruche) and Medieval Latin ostrica, ostrigius, all from Vulgar Latin avis struthio, from Latin avis "bird" (from PIE root *awi- "bird") + Late Latin struthio "ostrich," from Greek strouthion "ostrich," from strouthos megale "big sparrow," the first word perhaps from PIE *trozdo- "thrush" (see thrush (n.1)).
The Greeks also knew the bird as strouthokamelos "camel-sparrow," for its long neck. Among its proverbial peculiarities are indiscriminate voracity (especially a habit of swallowing small bits of iron and stone to aid digestion), a supposed want of regard for its eggs (which are incubated partly by the heat of the sun), and a tendency to hide its head when pursued. Ostriches do put their heads in the sand, but ostrich farmers say they do this in search of something to eat.
Like the Austridge, who hiding her little head, supposeth her great body obscured. ["Something written by occasion of that fatall and memorable accident in the Blacke Friers on Sonday, being the 26. of October 1623"]
Hence expressions cruel as an ostrich (late 14c.); foolish as an ostrich (late 15c.). From the Vulgar Latin word also come Spanish avestruz, Italian struzzo, Old English struta, German Strausz, Dutch struis, Danish struds.
For generic use of "sparrow" for "bird," compare Spanish pájaro, Romanian pasăre "bird," from Latin passer "sparrow."