It's an old-fashioned and somewhat literal translation. The Greek of, for instance, Luke 1:78 is "bowels of mercy". This is by no means an Orthodox problem alone, there were certainly some interesting early English translations. The thing was that the bowels were seen as the seat of mercy, as it were, or something like that. One common translation that avoids the whole imagery is "tender mercies", but that caused one Byzantinist who looked at a friend of mine's side-by-side Greek translation of some text to comment that Achilles sure spilled a lot of his opponents' tender mercies...