'underpack' originates from the combination of the prefix 'under-' meaning 'below or insufficient' and the verb 'pack' meaning 'to fill or load'.

'underpack' evolved from the Old English word 'under' and the Middle English word 'pakken', eventually forming the modern English word 'underpack'.

Initially, it meant 'to pack below capacity', but over time it evolved into its current meaning of 'packing insufficiently'.