'plane-sided' originates from English; it is a compound of 'plane' and 'sided', where 'plane' ultimately comes from Latin 'planus' meaning 'flat' and 'side' from Old English 'sīd' meaning 'side'.

'plane' entered English via Old French/Latin (from Latin 'planus') and came to mean 'flat'; 'side' comes from Old English 'sīd' (from Proto-Germanic) meaning 'side' or 'long edge'. The compound 'plane-sided' is a later English formation combining these elements to describe objects with flat sides.

Initially, the components meant 'flat' ('plane') and 'side' ('side'); combined, the compound evolved to mean 'having flat sides' in technical and descriptive contexts.