a) Adjective used to describe a moody or grumpy person
b) Can also be used as 'mard' as a noun to describe someone who is currently in a mood
c) Someone who is reguarly mardy can be described as a mardy bum or mardy arse
"Tom's in a mard with me because I called his mum a slag"
"Jenny never stops complaining - she's such a mardy bum!"