九牛一毛= a drop in the bucket or ocean It means that something which is very small that it cannot change anything or make any noticeable diference . It is like a unnecessary things. It is a very small part of whole or a big thing.

九条牛身上的一根毛。比喻极大数量中极微小的数量，不值得一提。（如果有什么疑问，可以问我)（if you have any questions,you can just ask for me..）

九头牛身上的一根毛。指极大数量中极微小的数量，微不足道 for example:这些钱对于他来说只是九牛一毛 means he has a lot of money and this amount for him means nothing