A more forceful variation of the Scottish omnipurpose exclamation "Ach!", usually followed by a complaint (1) or expression of hardcoredness(2). as many apostrophes or extra syllables and letters as the user feels necessary or amusing(3). This spelling is generally used to prevent it's being read as "Bach" in text, as it is very different from the famous classical musician, who was not at all scottish.
(1) "Bacgh! I hate rush hour traffic."
(2) "Bacgh! Bungee cords are for pansies, McCloud. Just jump already.