The author of The History of Galloway (1841) suggests Morrow or Murray is a corruption of Moor, “who from his swarthy complexion was called Black Morrow”. See The history of Galloway, from the earliest period to the present time by Mackenzie, William, of Galloway; Symson, Andrew, 1638-1712
“King James II of Scotland restored the lands when Sir Patrick’s son, Sir William, killed Black Morrow, a supposedly”Moorish” bandit living near Kirkcudbright. Sir William carried the head of the brigand to the king on the point of his sword. When King James demurred regarding the restoration of the estate, Sir William displayed the brigand’s head on his sword and advised the King to ‘Think on this’. This is the origin of the clan crest of this family and the Think Onmotto.” See Clan MacLellan
“James was somewhat astonished at the display, and was glad that the robbers had been disposed of, but forgot the promised reward whereupon Maclellan asked his Majesty to “Think on!” These words, with a Moor’s head transfixed on a sword, have since formed the motto and arms of the Maclellans. After the fall of the Douglases the family of Bombie acquired some of their possessions, and again became one of the most influential families in Galloway.” See KIRKCUDBRIGHT CASTLE AND THE LORDS KIRKCUDBRIGHT.
In Gaelic, “Black Morrow” becomes “Mor Dubh.” This is the shape in which the name appears in the quaint letter of John Elder, the “Redshank” priest, to King Henry VII., often quoted by Scottish Antiquaries, and printed at length in the Collectanea de Rebus Albanicis. He brings it in the following passage:–“Now, and pleas your excellent Maiestie, the said people land which inhabited Scotland afoir the incummyng of the said Albanactus (as I have said), being valiant, strong, and courageous, although they were savage and wild, had strange names, as Morewhow i. Mordachus; &c.” Mr. Skene spells it “Morrdhow;* but either spelling will stand for dubh, which in certain conjunctions, and in different accents is pronounced doo, hoo (the whow of Elder), and yew. The black warrior of the West Highland tales, Mordubh, is plainly another of this race. Scottish history (using the word “Scottish” in its widest sense) is full of suggestions of conflicts and even alliances between black, brown, and white races. See Ancient and Modern Britons: A Retrospect, Volume 1 By David MacRitchie
“Murdoch is the modern form. It is probable that in cases where the surname Murray was never written with qualifiying de before it, an actual descent from Moray-men or Moors is possessed by the bearer of it. The names More, Mure, Muir, Moir, and Morison have clearly arisen from a connection with the “Moor,” but not necessarily one of kinship. Several families so named “carry three Moors’ heads in their armorial bearings, as having some allusion to their name.” It seems likely that these forms are variations of the Gaelic genitive of Mor, and really signify de Moravia. Obvisouly, the heads would be borne by conquerors, not descendants, of Moors. It is also stated in Anderson’s “Scottish Nation” (whence the foregoing extract is taken) that the name More is derived from “the Gaelic Mor, big or large.” In all modern instances this must certainly be the case, but, at the first, it must have been derived from the Mor people. That the adjective itself also came from that source is a theory quite defensible, as I shall endeavour to show in another place. Anderson further says: “In a note [to a history of one of these families] the editor, William Muir, says, ‘The surname More certainly occurs very early in all the three British kingdoms, and is most probable of Celtic origin.” Such names as Mor-ton, Mor-ham, More- battle, may have come from this word when it meat a people and not moor-land. So also Morville, in Picardy. Picardy itself suggests the Piccardach (Picts). as does the old name for Poitiers-Pictavium.” See Ancient and Modern Britons: A Retrospect, Volume 1 By David MacRitchie