。
Bushy eyebrows were a distinguishing feature of his face.
Literal
Hair-deep eyebrow [topic-は] his [possessive-の] face [possessive-の] characteristic was.
毛深い ('hairy, thick-haired') is a compound i-adjective built from 毛 ('hair') + 深い ('deep'), where 深い means 'thick, dense' rather than literally deep. Similar compound adjectives include 用心深い ('cautious,' lit. 'caution-deep') and 注意深い ('careful, attentive'). まゆ毛 ('eyebrow') is itself 眉 ('brow') + 毛 ('hair').