Euphemism describing the act of defecating, metaphorically speaking. Otherwise known as taking a good hard shyte for oneself. This particular expression is used when the excrement is of a particularly large and solid nature.
Said in response to someone saying thanks. Comes from a Joke where David Hasslehoff asks a Clerk to change his name to the Hoff... when the Clerk said Sure, No Hassle. Now instead of saying "No Hassle" people just say Hoff.