Marrow can mean

• bone marrow, the interior of long bones
• vegetable marrow, a variety of squash, or a large courgette (zucchini).
• Marrow, a character in the X-Men comic series.
• Marrow (novel), Sci-Fi Book written by Robert Reed.
• Marrow (friend), mate, one of a matching pair. (As in Cockney rhyming slang: "Me old cock sparrow").