Originally Posted By: tsuwm
yeahbut, sense 1 (see above), being the earliest, looks like this:

1. Absolutely secure, or safe, from danger or interference.

Even if the OED is right and it doesn't refer to the bird, a more plausible explanation than the spigot or locking device would be that it is the opposite of the saying "to go off half-cocked," which refers to the firing of a musket and dates to about the time of the English Civil War in the mid 17th Century. To be cock-sure would be to be certain that the gun will fire because it is cocked properly.

Last edited by The Pook; 10/13/2008 3:47 AM.