Underlay

In the context of gambling, underlay is a slang term for a bad, unprofitable bet. It means that the odds on an event or a game are highly in the favor of the house. 

Games like craps and roulette are famous for having bets of vastly differing fairness to the player. For example, while craps is considered to be, in general, one of the fairest casino games you can play, the house edge for an any seven (big red) bet is a whopping 16.7%.

Underlay:

• A bad, unprofitable bet.