To chuff is to breathe with an audible puff sound. You might chuff in the cold air as you jog down a frozen winter street.
Anyone who huffs and puffs can be said to chuff, although the old-fashioned verb is often used to describe the sharp puffing sound made by a steam engine. You might read in a novel, "She heard the train chuff as it pulled out of the station into the night." The word chuff is imitative of the sound itself and dates from the early twentieth century. In British English, chuff has an entirely different meaning, "pleased or happy."