flyflyflysflies2noun (pluralflies)1a small insect with two wings2(British alsoflies)the part where you fasten a pair of trousers at the front: Your fly is undone!flyflyflysfliesflewflown1verb (flies, flying, flew/, has flown)1to move through the air: In autumn some birds fly to warmer countries.2to make an aircraft move through the air: A pilot is a person who flies an aircraft.3to travel in a plane: I'm flying to Abu Dhabi tomorrow.4to move quickly: The door suddenly flew open and John came in. A stone came flying through the window.