The characters combine literally to mean “many, like a cow's hair.” This creates a powerful and instantly understandable visual for something that is innumerable and impossible to count individually.
The idiom 多如牛毛 has its roots in classical Chinese literature, famously appearing in the *Book of Han* (《汉书》). In one story, an official laments that scholars possessing common skills were “as numerous as cow hairs” (多如牛毛), while truly loyal and exceptional individuals were “as rare as a qilin's horn” (麟角). This origin immediately establishes the idiom's slightly negative or dismissive undertone. It's not just about quantity; it's about the lack of distinction that comes with such abundance.
This is a very common idiom used in both written and spoken Chinese. It's versatile enough for formal essays and casual complaints.