guess ‧ ti ‧ mate /ˈɡestəmət, ˈɡestɪmət/ BrE AmE noun [countable] informal
[ Date: 1900-2000 ; Origin: guess + estimate ]
an attempt to judge a quantity by guessing it ⇨ estimate :
Could you give us a guesstimate of the numbers involved?
—guesstimate /-təmeɪt, -tɪmeɪt/ verb [intransitive and transitive]