Is it infeasible or unfeasible?
Score: 4.9/5 (75 votes) "Unfeasible" is in fact the more traditional, being the more popular of the two until "infeasible", for some reason, leapfrogged it in the late 1970s. Above is the British usage. (Both words have declined in popularity in American usage, but "infeasible" overtook "unfeasible"...