The term 'BIG BIRD THAT CAN'T FLY' can be summarized by the crossword solution EMU, which is a three-letter word. An EMU is a large, flightless bird native to Australia and the second-largest living bird by height after the ostrich. It is a member of the ratite family, which also includes the ostrich, rhea, and kiwi. Emus are known for their long legs and powerful strides, which allow them to reach speeds of up to 50 km/h.