Back in the day. Said by teenagers who haven't even had a day yet. Smell the coffee. Said by Vern, the ****.
my bet is that it isn't american in origin, it just became less used here. languages changes have parallels with the evolution of animals and plants. how do you deal with forgotten and misbegotten? forgetted and misbegetted? not to mention ill-gotten gains. i'll think of more later.
i'm going to offer up the three weasel -phobia words that their users use to denote hatreds rather than excessive fears. two of them are fears of particular prepositions/prefixes and one is a fear of the most peaceful set of organisations in the history of humanity.