E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Cock the Ears (To).
To prick up the ears, or turn them as a horse does when he listens to a strange sound. Here “cock” means to turn, and seems to be connected with the Greek o, a circle, and the verb ó.
o
, a circle, and the verb 
.
