To Endure
To Conceal, To Hide
transitive verb, godan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means endure so the verb vocab version is to endure.
Those who are able to endure are those who have had a lot of training. Who has had the most training of anyone? Ninjas. What group are they part of? The shinobi (しの).
父さん、どうして僕たちはアイツらの侮辱を忍ばなきゃならないの？
Why do we have to endure their insults, dad?