spoiling someone rotten

To spoil someone rotten is to indulge them excessively with gifts, attention, or leniency—pampering them to an extreme degree.

The phrase suggests the spoiling has gone so far it might corrupt or weaken the person's character, though it's often used affectionately: Grandparents are supposed to spoil their grandkids rotten.

The *rotten* intensifies what *spoil* already implies.