Too sure of what you know or what you can do, in a way that bothers other people. You talk as if you cannot be wrong. This is not quiet confidence; it is certainty with a strut. The cock here is probably the rooster — the bird that walks as if the yard belongs to him — plus sure, from the early 1500s. The word feels a little old and a little bookish now; in everyday talk most people say cocky instead. You still meet cocksure in writing and in British English. A cocksure answer can be right, which is part of why it stings: the tone, not only the facts, is the problem.