Your mum in a bag 

An insult originally coined in the mid 1700's by Anglo-Irish writer Jonathan Swift. The insult has no real defined meaning but It can be determined from context to essentially be accusing ones mother of being a possession. As was commonplace then and now many people carry their possessions in bags, thus the insult begins to make sense.

While not commonly used, the insult has begun to make an appearance on many social media sites such as facebook, instagram, imgur, youtube etc.

"I know your mother well. She's in a bag" or "Your mum in a bag"

Your mum in a bag by JewshWillmort February 27, 2019