Mary Hinge is a Spoonerism (or very nearly) for Hairy Minge.
A Spoonerism is where the initial letters (or syllabols) of two words, as in "Mary Hinge" becomes "Hairy Minge", are accidentaly swapped to make two more words, usually with a comic outcome. Sometimes the Spoonerising of two words deliberately is to diguise rude sayings in mixed company. i.e. Sweaty Bollocks become Betty Swallocks. Simple!
by Favey Dyfe March 19, 2010