'guesstimate' originates from a blend of the words 'guess' and 'estimate', where 'guess' meant 'to form an opinion or conclusion about something without sufficient information' and 'estimate' meant 'to roughly calculate or judge the value, number, quantity, or extent of something'.

'guesstimate' was coined in the mid-20th century as a portmanteau of 'guess' and 'estimate', reflecting the informal nature of the term.

Initially, it meant 'a rough estimate', and this meaning has largely remained the same in modern usage.