drop in the bucket; drop in the ocean; insignificant fraction
Idiomatic yojijukugo meaning something is so small compared to the whole that it is negligible, like a single hair from nine oxen. Often used in expressions of modesty or to emphasize insignificance.
彼の寄付は全体から見れば九牛の一毛に過ぎない。
His donation is just a drop in the bucket compared to the total.
My knowledge is but a drop in the ocean; I still have much to learn.