fell of的原形是fall off，它是fall off的过去式。表示“落下”、“跌倒”、“减少”等意思。如：

Her cap fell off in the street and blew away.她的帽子掉在大街上，被风吹走了。

When she said she was engaged I nearly fell off my chair.她说她订了婚时，我几乎大吃一惊。