bag of spiders
Describes a person or item as largely problematic and should be avoided as much as possible. Can be imagined as if you were not keen on spiders but were given a bag of spiders: Opening the bag would mean the spiders would immediately escape and be everywhere. Not something to be desired.
Don't date Jim's ex, she's a real bag of spiders.
bag of spiders
Just imagine someone holding a bag of spider, it doesn’t make sense
He’s a bag of spiders