This word is capable of at least two interpretations.
1. A significant word in the Third Degree according to the French Rite and some other Rituals (see Mac).
2. In the Order of the Beneficent Knights of the Holy City, the Recipiendary, or Novice, is called Macbenac.
Masonic writers have generally given to this word the meaning of “is smitten,” deriving it probably from the Hebrew verb macha, to smite. Others, again, think it is the word mak, rottenness, and suppose that it means “he is rotten.” Both derivations are, in Brother Mackey’s opinion, incorrect. Mac is a constituent part of the word macbenac, which is the substitute Master’s Word in the French Rite, and which is interpreted by the French ritualists as meaning “he lives in the son.” But such a derivation can find no support in any known Hebrew root. Another interpretation must be sought. Doctor Mackey believed there is evidence, circumstantial at least, to show that the word was, if not an invention of the Sentient or Dermott Freemasons, at least adopted by them in distinction from the one used by the Moderns, which latter is the word now in use in the United States of America.
Brother Mackey was disposed to attribute the introduction of the word into Freemasonry to the adherents of the House of Stuart, who sought in every way to make the Institution of Freemasonry a political instrument in their schemes for the restoration of their exiled monarch. Thus the old phrase, “the Widow’s Son,” was applied by them to James II, who was the son of Henrietta Maria, the widow of Charles I. So, instead of the old Master’s word which had hitherto been used, they invented macbenac out of the Gaelie, which to them was, on recount of their Highland supporters, almost a sacred language in the place of Hebrew. Now, in Gaelic, Mac is son, and benach is blessed, from the active verb oeannaichy to bless.
The latest dictionary pushed by the Highland Society give this example:
“Benach De Righ Albane, Alexander, Mac Alexander,” etc., that is, Bless the King of Scotland, Alexander, son of Alexander, etc. Therefore we find, without any of those distortions to which etymologists so often recur, that macbenac means in Gaelic the blessed son. This word the Stuart Freemasons applied to their idol, the Pretender, the son of Charles I.
the source: Mackey’s Encyclopedia of Freemasonry