1.
Rate something lower than its
real/actual value , under appreciate.
2.Estimate/ rate someone lower than his/her actual potential, qualifications.
You underestimated that problem because you are not familiar with it.
Don't underestimate 'em simply because they are young.
Julian: Ask your lil bro to quit that chalenge it is very hard for him.
John:
Don't underestimate my little
brother his able.