Reportedly from the practice of substituting a butchered cat, etc. for a suckling pig when sold in a sack at a market. Other theories circulate but all involve the purchase of a cat in a bag, where the buyer expected better.
- (idiomatic) Something whose true value is concealed or unknown and may be lower than expected by the buyer or claimed by the seller, especially something offered for sale.
something whose true value is concealed or unknown