加介词at，为固定词缀搭配，中文意思是嘲笑因……而发笑

She knew that the girls used to imitate her and laugh at her behind her back.她知道那些女孩子过去常在背地里模仿她、嘲笑她。

When I was a lad his age I would laugh at the strangest things.

我是他那么大的小伙子时，我常嘲笑一些最奇怪的事情。