Green’s Dictionary of Slang

marley slopper n.

[marley n.]

one who is splayfooted.

Walford’s Antiquarian 252: A marley-slopper is a splay-footed person. Marley is a corruption of marble... In playing it is common for a boy to put his heels together, and turn out his toes... to stop an eccentric marble [F&H].