His unibrows are so thick that they look like a single line across his forehead.

unibrows 是复合名词，由前缀 uni-（表示"单一"）和名词 brows（眉毛的复数形式）组成，字面意思是"单一的眉毛"，指两条眉毛连在一起形成一条连续的眉毛。