在家玩刀英文可以说作 "playing with a knife at home"。
这个短语的来源是指小孩在家里玩耍时使用刀具，这是不安全的行为。在英文中，"playing with a knife" 表示使用刀具玩耍，"at home" 则指在家里。
下面是一些使用这个短语的英文例句，并附有中文翻译：
"It's not safe to play with a knife at home. You could get hurt." (在家里玩刀不安全。你可能会受伤。)
"I saw Johnny playing with a knife at home and had to tell his mom." (我看到约翰尼在家里玩刀，就不得不告诉他妈妈。)
"Don't play with a knife at home. It's dangerous." (不要在家里玩刀。这很危险。)
总之，"playing with a knife at home" 指小孩在家里使用刀具玩耍，这是不安全的行为。