You would think a dude named after a jug would be kind of dim... and you'd be right.
Flask, the third mate on board the Pequod, is a "short, stout, ruddy young fellow, very pugnacious concerning whales, who somehow seemed to think that the great Leviathans had personally and hereditarily affronted him" (27.4).
Flask, who is from a town on Martha’s Vineyard (an island off the coast of Massachusetts) called Tisbury, is a small, fierce, practically-minded fellow who doesn’t think too deeply about anything and butchers whales with great (and creepy) enthusiasm.
His practicality sometimes prevents him from noticing the deeper connotations of things: when all the characters are busy interpreting the symbolic meaning of the gold doubloon, all Flask sees is money. Flask is also a little bit naive, and Stubb occasionally mocks him by getting him to believe ridiculous stories.
His nickname is "King-Post," because he seems just like "the short, square timber known by that name in Arctic whalers"—a piece of wood that braces the ship and keeps it firm and steady (27.4).