Once a use and ever a custom.

A single act may establish a pattern: what begins as an exception or experiment can come to be treated as the usual way of doing things.

The proverb warns that allowing something once can make it harder to refuse later, because people may come to expect it again.

Use it to caution against a one-time exception when the concern is that others will expect the same allowance in the future. It can also describe how a new routine takes hold.