Colloquial contraction of ばかり, used in casual speech to emphasize limitation or exclusivity. Often carries a nuance of 'all the time' or 'nothing but'.
See also: ばかり
彼はゲームばっかしてる。
He does nothing but play games.
My kid eats nothing but sweets.
It's been nothing but rain lately, huh.
弟は遊んでばっかで宿題をやらない。
My little brother does nothing but play and never does his homework.