Come off the grass/come off it

Usually used in the form of an exclamation and means, don’t exaggerate or don’t tell lies and generally expresses incredulity. It dates in this sense from the early 19th century. Dickens used the expression in Martin Chuzzlewit (1843). The meaning of grass in this context remains obscure but perhaps by “keeping off the grass” metaphorically, one ceases to transgress. It is often shortened to come off it. See also Do me a favour.