Mustard是“芥末”的意思，keen是“渴望、热切”的意思，as keen as mustard字面意思像芥末一样刺激，引申为非常渴望；非常热情；特别感兴趣。

【例句】
His family is very poor, but he wants to go to school as keen as mustard.
他的家庭特别穷，但是他特别渴望去上学。

She is as keen as mustard to go to the park, because her friends are waiting there for her.
她非常热切地想去公园，因为她的朋友们正在那里等她。