วัวลืมตีน

Etymology
From ; literally "a cow that forgets its own feet".

Noun

 * 1)  a person who forgets or denies his background, or changes his attitude or behaviour into an unfavourable or unpleasant one, after acquiring a better status, condition, etc.

Usage notes
Sometimes used in combination with the expression คนลืมตัว (kon lʉʉm dtuua; literally "a person who forgets his own self"), as.