Mard

"Mard" is an old British circumlocation for "shit" that comes from the French "merde" = "shit". As used nowadays as a noun it means a sulky, petulant, ill-tempered attitude, or a person with that attitude.

In that use it's probably a contraction of "mardy bum" (= "shitty ass"), a synonym used in the Arctic Monkeys' song of the same name.

Well, now then, mardy bum
I've seen your frown and it's like looking down
The barrel of a gun.