Moan bag

n.

A continually vocal pessimist.

In use

Yer wan is some moanbag, she's always complaining about her rasher arse.

Let Lawrence Muck translate it to posh English for you
Lawrence Muck

Hullo there. Would you like me to translate 'Moan bag' in use for you?

That woman is an inveterate complainer; she is forever grumbling about her posterior.