'planus' originates from Latin, specifically the word 'planus', where the root 'plan-' meant 'flat' or 'level'.

'planus' remained in Classical Latin as 'planus', continued in Medieval Latin and New Latin, and was later adopted into English usage primarily in medical, anatomical, and scientific terms (for example, 'pes planus').

Initially, it meant 'flat' or 'level' in Latin, and this basic meaning has been retained; in English it is used mainly as a technical or scientific term meaning 'flat'.