Heaven Forbid Meaning

(idiomatic) To say that one hopes that something does not happen.

Example: 1883, Howard Pyle, The Merry Adventures of Robin Hood Chapter V
  "Now, by Our Lady's grace," cried the Butcher, "well do I know thy name, and many a time have I heard thy deeds both sung and spoken of. But Heaven forbid that thou shouldst take aught of me! An honest man am I, and have wronged neither man nor maid; so trouble me not, good master, as I have never troubled thee."
  "Nay, Heaven forbid, indeed," quoth Robin, "that I should take from such as thee, jolly fellow! Not so much as one farthing would I take from thee, for I love a fair Saxon face like thine right well—more especially when it cometh from Locksley Town, and most especially when the man that owneth it is to marry a bonny lass on Thursday next. But come, tell me for what price thou wilt sell me all of thy meat and thy horse and cart."