The great Oxford English Dictionary waxes facetious about the word cocksure.
It says, in effect, this should mean “as sure as a cock,” that is, “as secure, safe, certain, trustworthy, reliable, etc.,” as a cock.
But what kind of cock? Certainly not a rooster.
So, the dictionary suggests, possibly the reference, four and a half centuries ago, “may have been to the security or certainty of the action of a cock or tap in preventing the escape of liquor, or perhaps of a cock with a removable turning-key in leaving the contents of a tun secure from interference.”