To be very unwieldy or unmanageable; to be nearly impossible to organise. Usually said of a group of people. 

In case you didn’t know, an idiom is a phrase that has a figurative meaning. These phrases, or sayings (or proverbs if they are longer), don’t make literal sense – they don’t exactly mean what the words say – but work instead as ‘pictures’. Clishmaclaver used this very idiom of herding cats when trying to chaperone BHS pupils safely through the London underground a few years ago..! 😉

Art credit: ©rogerdahl.aol.com