underrate, underate, under eight

Underrate or underrate means to underestimate; to fail to regard realistically--
If you underrate your opponent, you are more likely to lose.
Be careful not to underrate the competition.

Underate or underate ( one 'r' ) is the past tense ( yesterday, or some time ago ), of "undereat", meaning to fail to consume adequate food or nourishment--
The baby underate for the first few months, and occasionally lost weight.
He always underate when he worked overtime, and came home feeling miserable.

Under eight, under eight, Under 8, or under 8, means fewer than eight or 8 in number ( something you count ), or less than eight or 8 in quantity ( something you measure or weigh )--
This game is for children under eight years old.
This game is for children under 8 years old.
This dress should fit a baby under eight pounds.
This dress should fit a baby under 8 pounds.

Now that you know that, you can say--
"Don't underrate the nurse's advice; our infant son often underate, and was still under eight pounds when he was four months old."