'tortoiseshell-patterned' originates from the modern English compound 'tortoiseshell' + the adjective-forming past-participial suffix '-patterned'. 'tortoiseshell' itself derives from 'tortoise' + 'shell'; 'tortoise' entered English via Old French 'tortue' (meaning 'turtle') and related medieval forms.

'tortoiseshell' appeared in Middle English as a compound referring to the shell material and objects made from it; over time it extended to describe the distinctive mottled color and pattern. The element '-patterned' is formed from the noun 'pattern' + the past-participial/adjectival '-ed' to mean 'having a pattern of...'.

Originally it referred specifically to objects made from or resembling the actual shell material of tortoises/turtles; over time it broadened to describe any item that displays a similar mottled color pattern (i.e., a decorative/color description rather than a material description).