noun

• (uncountable) The substance inside bones which produces blood cells.
  "Turning back, then, toward the basement staircase, she began to grope her way through blinding darkness, but had taken only a few uncertain steps when, of a sudden, she stopped short and for a little stood like a stricken thing, quite motionless save that she quaked to her very marrow in the grasp of a great and enervating fear."
• (figurative) The essence; the best part.
  "It takes from our achievements[…]/ The pith and marrow of our attribute."
• (figurative) The inner meaning or purpose.
• (countable, medicine, colloquial) Bone marrow biopsy.
  "This patient will have a marrow today."

noun

• (Geordie, informal) A friend, pal, buddy, mate.
  "Cheers marrow!"
• (mining, slang, obsolete) A miner's mate or assistant.
  "A 'getter' or miner is paid 1½ to 2 cents per hundred weight of Coals excavated, […] but out of this sum, his "marrows" or assistants who do the business of 'putting' and 'hurrying' for him must be paid […]"
• (Scotland or archaic) One of a pair; a match; a companion; an intimate associate.
  "The moon’s my constant Mistresse / & the lowlie owle my morrowe. / The flaming Drake and yͤ Nightcrowe make / mee musicke to my sorrowe."