'shaly' originates from English, specifically from the noun 'shale' plus the adjectival suffix '-y', where 'shale' referred to a type of fissile sedimentary rock (shale).

'shaly' developed in post-medieval English as an adjective formed from Middle English noun forms of 'shale' (e.g., Middle English 'schale'/'shale') and the productive adjectival suffix '-y', eventually stabilizing as 'shaly' in modern English.

Initially it meant 'of or relating to shale' and over time has retained that core sense while also being used more generally to describe layered or fissile textures in rocks.