1. To spend a large amount of money. (When speaking of one's own actions.) My sister loves to shop. She's always out looking for new ways to lighten her wallet.

2. To take money from someone; to cause someone to spend a significant amount of money. (When speaking of someone or something acting on another person.) Sure, it's a nice hotel, but it will lighten your wallet faster than you can say "overpriced." I'm always wary of souvenir merchants in foreign cities, whose sole objective is to lighten travelers' wallets.