E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Parturient montes, nasctur ridiculus mus. The Egyptian king Tachos sustained a long war against Artaxerxes Ochus, and sent to the Lacedemonians for aid. King Agesilaos went with a contingent, but when the Egyptians saw a little, illdressed lame man, they said: Parturiebat mons; formidabat Jupiter; ille vero murem peperit. (The mountain laboured, Jupiter stood aghast, and a mouse ran out.) Agesilaos replied, You call me a mouse, but I will soon show you I am a lion.