Bashful is one of those wonderful words without an opposite. Try as you might, nobody will ever call you bashless. Which poses the question: what the devil is bash and how did you get so full of it?
To answer that question one can turn to the Old Curiosity Shop where Dickens mentions that Dick Swiveller "spends all his money on his friends and is Bah!'d for his pains."
By this Dickens means that Swiveller's friends say Bah to him, Bah being an exclamation of contempt. Dickens looks terribly original, but in fact bah has (probably - this is all merely the best theory) been verbed before, by the French and Normans a thousand years ago.
First, you should note that bah comes straight from the French. But the Normans had a verb ebahir, which meant to be astonished, or more literally to be reduced to saying 'bah' in amazement. This got altered to abaïr, and that got altered to abaissir and that got altered to abash.
These days it's easy to be abashed in the passive; but once upon a time it was just as common to go around abashing people until they were abashed, or just bashed, at which point they became bashful.
Thus, by a preposterously serpentine route, Dick Swiveller is abashed.
None of which has anything to do with a bash on the head, which appears to come from the Old Norse who had a deplorable habit of doing a lot of such bashing.