Luff

The luff has two meanings. As a noun, it is the leading (forward) edge of a sail. As a verb, it describes the fluttering of a sail when it loses pressure — typically because the boat is sailing too close to the wind. Sailors say a sail is 'luffing' when its forward edge is shaking. The phrase 'head up until you luff, then bear away' is the basic method for finding the highest course a boat can sail close-hauled.