To Be Careful
Transitive verb and Godan verb
つつしむ
電車の乗客は、携帯での会話を慎むべきだ。
People on the train should refrain from having cell phone conversations.
If you have humility you will try to be discreet.
Why are you going to be discreet? Because that tutu she (つつし) wears is really embarrassing.