from the Greek οστρος for gadfly or mad impulse
(derivation: estrous cycle, hormonal changes associated with ovulation)

Oestrus refers to the gadfly that Hera sent to torment the nymph Io
after she was impregnated by the immortal Zeus (while in the form of a heifer).

Plato used "oestrus" to describe a soul "driven mad by the gadfly of desire".


http://dictionary.reference.com/browse/oestrus