①undermine one's authority.
If you point out your boss's mistakes in front of people, you are undermining his or her authority.
If you undermine someone's authority, it means to do things that that make people lose respect for them.
②do crunches = half sit-ups
腹筋の半分の動き。おでこを膝に付けるまで上体を起こすのが日本語で言う腹筋運動で、おなかだけでなく、肩、背中などの上半身全体の筋肉を使う運動。
crunchesは上体を腹筋の半分ほどまでしか起こさないため、腹筋しか使わない運動。