From Dicio: “achismo (achar+ism) – Tendency of evaluating situations according to one’s own opinions or intentions, many times unjustified.”
Dicionário Informal: “Culture of making groundless comments, without knowledge, deducing by personal experience something that you’re not sure of.” Examples are when people say things like: “Acho que não se deve correr riscos desnecessários, acho que vai dar certo...”
We all know the verb “Achar”, but did you know it has an “-ism”? On WordReference, I found some possible creative suggestions for translating it into English, one was “as-we-now-knowism” and “as-I-now-thinkness”. On Orkut, I found someone suggesting “armchair quaterback” (which means “a person who offers advice or an opinion on something in which they have no expertise or involvement”) and another person simply said “assumption”. Linguee, on the other hand, suggests “guesswork”. Perhaps something more along the lines of “presumptuousness” or the made-up “assumptivness” would work, too.
The problem with most of the Portinglês suggestions is that none of them speak of a tendency to do such things. In the comments of this post, “self-opinionated” was suggested. Fits pretty well.