使劲-make an effort 吃奶的劲是用比喻的方式形容使了很大的劲-make a great effort. 句子的意思是，尽自己很大的努力也没有牛跑的快。 原文应该是：他使出了吃奶的劲也赶不上牛的速度。

他使出了吃奶的劲也比不上牛的速度。 1. He cannot catch up with the bull even using all the strength he has. 2. 使 verb use 劲 noun strength 使出了吃奶的劲 means he tries his best

吃奶的劲： means this people use all power to do one thing ( when you are young ,you must drink breast milk，so you must drink more and use all power to drink it, when you older,you want to do one thing,you use all power,so people like speak you use power that drink breast milk. so this sentence talk about one people use all power still can not faster bull

使劲，必须一起用，单独用就没啥特别意思了，使劲意思是“用力”，很像 英文“ exert all one's strength”

In fact, the phrase, that“使出吃奶的劲儿”, is old-fashion.