'insides' originates from Old English elements 'in' and 'sīd(e)', where 'in' meant 'in' or 'within' and 'sīd' meant 'side, region'.

'inside' developed in Middle English from Old English combinations (e.g. 'inneside' / 'insīde') and later formed the plural 'insides' for internal parts or organs.

Initially it referred to the inner side or inner area ('on the inside'); over time it also came to refer specifically to internal parts or organs and figuratively to inner feelings.