This word is derived from the double entendre/ambiguous parsing of the phrase "shit shitting shit". This phrase can be parsed in
two ways.
The first interpretation requires reading the phrase as "shit-shitting shit", where "shit-shitting" is an adjective and "shit" is a noun. This is analogous to "flesh-eating
virus".
The second interpretation essentially means the same
thing, but is parsed in a different order: the first "shit" is "shitting shit", analogous to "virus eating flesh".
There are
two parses, each containing three shits. Thus, there are
six shits in this phrase.