The Americans have some great words, for example blowhard. Can't think of a UK English equivalent, but opinionated twit conveys the sense. This is a behaviour pattern that nimrods seem especially prone to, unfortunately. Have a look at Consimworld's GMT forum for recent examples - various blowhards condemning GMT's upcoming Decision at Kasserine on the basis of nothing more than a preview of the counters. I'll say it again - blowhards!