Cleg is the north country term for horseflies, deriving from ON kleggi (cf. Mod. Norwegian klegg).
Horseflies are ugly, vicious ,bloated, biting insects which suck the blood from exposed parts of
fishermen, usually in Scotland or Ireland. They are impossible to deter with insect repellent;
insect repellent attracts them. They should be smacked dead before they bite, if possible.
Cleg-smacking is an alternative pursuit to fishing during long, hot, sweaty and windless days when
the fish are as sulky as the weather (McCully, 1993).
Courtesy of Ben Spinks