'Marra' is actually the local pronunciation of 'marrow', a vegetable, our name for zucchini. Why or how the name of this vegetable came to be used to mean 'mate' or 'partner' in NE England I have been unable thus far to ascertain. Eric Partridge has no entry on it in his Dict of Slang & Unconventional English, and Chambers Dict gives it as a separate entry, 'origin unknown'. Anyone from Tyneside/Wearside [or elsewhere] happen to know?

~M~