guesstimate

 
(also guestimate)
informal

noun

Pronunciation: /ˈgɛstɪmət/
  • an estimate based on a mixture of guesswork and calculation: a rough guesstimate of public expenditure

verb

Pronunciation: /ˈgɛstɪmeɪt/
[with object]
  • estimate (something) using a mixture of guesswork and calculation: the task is to guesstimate the total vote

Origin:

1930s: blend of guess and estimate