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from half + marrow, of obscure origin.
(From E-NED: The localities would seem to point to a Scandinavian etymology, but no possible Scandinavian source is known, unless indeed the sense of the Eng. n. can have been developed from that of ON. marg-r (lit. ‘many’) friendly, communicative. Phonologically this etymon would be admissible, as the word occurs so late that the absence of recorded forms with guttural causes no difficulty.)
From:Mr Rutherford's Letters
Volume the First, 1761
123. To Mr. J.R.