(Scotch) a mate, companion, friend. “Not marrow”- that is, not a pair. The Latin word medulla (marrow) is used in much the same way as “mihi hæres in medullis” (Cicero); (very dear, my best friend, etc.).
Busk ye, busk ye, my bonnie bonnie bride, Busk ye, busk ye, my winsome marrow.
The Braes of Yarrow.
“One glove [or shoe] is not marrow to the other.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894