The Collaborative International Dictionary of English v.0.48:
Cock \Cock\, n. [It. cocca notch of an arrow.]
1. The notch of an arrow or crossbow.
[1913 Webster]
2. The hammer in the lock of a firearm.
[1913 Webster]
At cock, At full cock, with the hammer raised and ready
to fire; -- said of firearms, also, jocularly, of one
prepared for instant action.
At half cock. See under Half.
Cock feather (Archery), the feather of an arrow at right
angles to the direction of the cock or notch. --Nares.
[1913 Webster]