>>265
He might be mishearing "You musta had nothing", the "a" on "must" being a contraction of "must've", itself a contraction of "must have". It's a subtle difference, but definitely one native speakers will notice. With the way Japanese omits vowels (not completely), it wouldn't be surprising if he's thinking マスト・ハーブ ー＞ マスト instead of マスタ, although they are definitely very different. If you say "He musta gone the other way.", it sounds fine; if you say "He must gone the other way" you're clearly a foreigner. I don't think I would even expect that error from a small child. Also, "musta" or "must've" isn't a mistake, it's just more colloquial than you'd probably learn in a language class; and you'd probably never write "musta" unless you were trying to give a character an accent.
Guy's a douchenozzle either way.