'underweigh' originates from the combination of 'under-' meaning 'below' and 'weigh' from Old English 'wegan', meaning 'to weigh'.

'wegan' transformed into the Middle English word 'weighen', and eventually became the modern English word 'weigh'.

Initially, it meant 'to weigh something', but over time it evolved into its current meaning of 'to weigh less than expected'.