A till is a cash register or drawer for keeping money in a shop or office.
When you have your hand in the till, you have easy access to the moneybox. The idiom have one’s hand / fingers in the till is used to suggest that an employee is stealing the money that belongs to his employer.
- How could he buy such a big house? He must have his hand in the till. (= He must be stealing money from the place he works.)
- They caught him with his hand / fingers in the till and promptly terminated his service.
- The manager was outraged when he found out that the cashier had his hand / fingers in the till.
The expression have one’s hand in the cookie jar also means the same.