'slate-gray' originates from English, specifically a compound of 'slate' and 'gray', where 'slate' referred to the fine-grained metamorphic rock and 'gray' referred to the color.

'slate' (the rock name) comes into English from older Germanic and possibly Old French influences for words describing split stone; 'gray' comes from Old English 'græg'. The compound 'slate-gray' developed in modern English to name the particular color resembling slate.

Initially it specifically described the actual color of slate rock; over time it broadened to refer to any similar bluish-gray shade used for fabrics, paints, and other materials.