topmast n the mast next above a lower mast and topmost in a fore-and-aft rig Usage(s) Readers of Commodore Hornblower will find it built on the same lines as its predecessors: its topmast in a cloud of fantastic thrills, its keel afloat in Royal Navy lore. On the way to Gloucester the fore topmast of Bluenose buckled. Even so, one sloop's topmast was sprung.