a bit better than; not much more than; just a step up from
Idiomatic expression used to belittle something by comparing it to a lower standard, implying it is only slightly better or barely different. Often carries a dismissive or sarcastic tone. Literally means 'with hair grown on it', suggesting a minimal improvement.
The food at that restaurant is just a convenience store bento with a little extra.
彼の英語は中学生に毛の生えたレベルだ。
His English is only a bit better than a junior high school student's.
From the literal image of something having grown hair, suggesting a minimal or superficial improvement. The exact origin is uncertain, but it is a long-established colloquial idiom.