Derived from classical form of the 'Wizard's Sleeve', Hagrid's Pocket being reserved for a rather hairy, large, loose and dirty form of the 'Man-hole'.
Further connotations in that Hagrid, not being a proper wizard, infers likewise on the quality of said pudenda.
holy lord, she was fine up-top, but I thought I put my hand in Hagrid's pocket when I ventured south!