What is Flask's nickname among the Pequod crew?

Flask’s nickname among the Pequod crew is King-Post.

Flask is the Pequod’s third mate, a short, stout sailor from Martha’s Vineyard. The crew compares his compact, powerful build to a king-post, a short upright timber used to support or brace parts of a ship or other structure. The nickname captures both his physical shape and the novel’s habit of describing sailors through the tools and materials of maritime work.

Flask approaches whaling aggressively, as though the whales have personally wronged him. That attitude contrasts with Starbuck’s caution and Stubb’s genial fatalism. His harpooneer is Daggoo, whose great height creates an additional visual contrast with Flask’s shorter stature. King-Post is therefore a character nickname, not Flask’s job title or a reference to command rank.