Bodyguard
Bouncer, Guard
noun
"Use your heart and this pole when you guard me," you tell your bodyguard. A good bodyguard needs both — the heart of a lion, and a nice sturdy pole to smack fools with.
用心 actually means "caution," so 用心棒 suggests someone who watches out for danger — often with a 棒 (stick) in hand! The word has a bit of an old-school, samurai-era feel, but it still gets used both in writing and conversation from time to time.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
私の用心棒をしてくれない？
Will you work as my bodyguard?
彼は用心棒として働いている。
He works as a bodyguard.
用心棒がいるから、安心して歩ける。
I can walk safely because there's a bodyguard.
彼女の用心棒は、いつも彼女の後ろにぴったりくっついている。
Her bodyguard is always right behind her.