A fictitious person (or entity) created to serve a legal purpose, but with no genuine substance, for example a shell company. The origin of the term strawman was in complex rules of pleading in the English courts, which meant that all cases had, by some contrivance, to fall within existing causes of action; thus someone seeking to bring a suit might need to arrange to be sued by a putative third party with a essentially specious claim (which would in fact be known by all to be invalid and without substance).
Such third parties were known as strawmen, because individuals offering to serve in this role “hung around” outside courthouses advertising themselves by displaying a symbolic strand of straw in their boot. Eventually the necessity to have an actual individual serve as the “strawman” fell into disregard and the strawmen became wholly fictitious. Subsequently pleadings reform led to an end to the need for such legal fictions and the term “strawman” came to be regarded as pejorative and generally implying some sort of chicanery.