John Havlicek, a relentless force for the Boston Celtics over two decades and two championship eras and one of the greatest clutch stars in N.B.A. history, died Thursday in Jupiter, Fla. He was 79. Havlicek showed an unassuming but unyielding consistency throughout a 16-season, Hall of Fame career. He was known by the nickname “Hondo,” given him by a childhood friend who had trouble pronouncing his surname and who thought Havlicek’s strong, silent demeanor was reminiscent of John Wayne in the 1953 movie of the same name. John Joseph Havlicek was born in Martins Ferry, Ohio, on April 8, 1940, the second son of Frank Havlicek, who had emigrated to the United States from Czechoslovakia at 12, and Mandy (Turkalj) Havlicek, who was of Croatian descent but born in the United States. His parents ran a general store, and the family lived above it, on U.S. 40 in nearby Lansing, an Ohio Valley town of a few hundred residents near Wheeling, W.Va. Havlicek liked to cut through an uphill wooded area on the way to meeting his friends, including Phil Niekro, who lived across the street and, along with a brother, Joe, went on to pitch in the major leagues. Havlicek maintained a low basketball profile after retiring from the Celtics, splitting his time between New England and Florida with his wife, Beth (Evans) Havlicek, whom he had met at Ohio State and married in 1967. An avid golfer, Havlicek continued the work he had begun during basketball off-seasons, as the vice president of the International Manufacturing & Marketing Corporation in Columbus, Ohio. He also started a company, John Havlicek All Sports Products, and owned Wendy’s franchises. Havlicek is survived by his wife; a son, Chris, who played basketball at the University of Virginia; and a daughter, Jill Havlicek Buchanan, a high school basketball and lacrosse player who also played lacrosse at Virginia and married Brian Buchanan, a former major league player.