means The act of slowly carving or shaving thin slices off a piece of wood with a knife, usually to shape it or simply to pass the time.

from From the Old English 'thwitan,' meaning 'to cut or pare,' which gave us the noun 'whittle' for a knife — a butcher's blade was once literally a 'whittle.' To 'whittle' first meant to cut with such a knife, and over time the word also drifted into the figurative 'whittle down,' as if you could shave a problem the way you shave a stick.