Learn English – Idiom for a person being kind to someone who doesn’t deserve it

Such a person can be said to have "forgot where he came from", meaning he's forgotten his humble roots and is acting as if he was born to wealth.

You can consider welch or welcher. These terms are used for people who fail to repay a small debt. The debt is usually a betting or gambling debt but these terms can be applied to other contexts as well. They are considered derogatory.

welch (n)

A person who defaults on an obligation, especially a small one.

She's a welch. That isn't hers, I lent her that watering can three years ago.

welch (v)

1. To fail to repay a small debt.
2. To fail to fulfill an obligation.

http://en.wiktionary.org/wiki/welch

Related questions:

• Are the terms "welsh" or "welch" (as in reneging on a bet) derogatory toward the Welsh people?
• What would you call someone who imposes on other people's generosity?
• What do you call someone who is always asking for favours?