A phrase used to describe having sexual relations with a homeless person or "hobo." The phrase originated from an Australian Folk Song of the same name, which tells the story of a lonely "swagman" (a man who travels the country on foot looking for work) who carries all his belongings in a "Matilda" (a particular term for a bag). He sings affectionately to his Matilda because it is the only company he has.
Man, you were trashed last night. You committed a Waltzing Matilda.