of Luff
Source: Webster's dictionaryIf the sheet is too loose the sail will flap in the wind, an occurrence that is called "luffing." Source: Internet
Optimum sail angle can be approximated by pulling the sheet in just so far as to make the luffing stop, or by using of tell-tales - small ribbons or yarn attached each side of the sail that both stream horizontally to indicate a properly trimmed sail. Source: Internet