spoil somebody rotten
spoil (one) rotten
To cause one to expect to receive everything they want by always giving it to them. In this phrase, "rotten" is an intensifier. I think some grandparents believe it's their duty to spoil their grandchildren rotten.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
spoil somebody ˈrotten(informal) give somebody everything they want or ask for: She spoils the kids rotten.
Farlex Partner Idioms Dictionary © Farlex 2017