Sorry, guest was me post-Secret-Santa-ing.

No, no idea what she meant, but it has been used throughout my lifetime. Her tone of voice said all that was needed.

'Mardy' in Yorkshire is used by children to children who cry easily, want things their own way, won't share or join in. "Mardy bum, Mardy bum, tell your mother to smack your bum" was our 1950s taunt. Presume mardyarse is just a cruder version taken up by adults.