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教育部國語辭典

妄自菲薄

= 自卑過甚、自暴自棄, 過於自卑而不知自重

Translation:

1. Sell oneself short

2. Degrade oneself

3. Belittle oneself

For example:

Don’t sell yourself short.  Be all that you can be.

不要妄自菲薄, 做你所能

http://idioms.thefreedictionary.com/sell+short

Don’t degrade yourself by cheating and telling lies.

不要貶低自我而欺騙說謊.

Never belittle yourself by lowering your standards.

不要妄自菲薄, 自貶身價而降低你的標準

By the way, there is a difference between 妄自菲薄 & 低估.

So “underestimate” would not be the right description for 妄自菲薄.