An expert who watches over and defends people at all hours is the nurse. That's what nurses do, 24/7. They have to check up on people, defend their health, and watch over them, making sure they're okay. What a tough job.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
The nurse faked a laugh to her patient.