from Greek οἶστρος (gadfly,mad impulse). Specifically, this refers to the gadfly that Hera sent to torment Io, who had been won in her heifer form by Zeus. Euripides used "oestrus" to indicate "frenzy", and to describe madness. Homer uses the word to describe panic. Plato also uses it to describe the soul "driven and drawn by the gadfly of desire".

http://en.wikipedia.org/wiki/Oestrous