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.
See also: rotten, spoil

spoil somebody ˈrotten

(informal) give somebody everything they want or ask for: She spoils the kids rotten.
See also: rotten, somebody, spoil