Quibbling; making your own argument good by forcing certain words or phrases from their obvious and ordinary meaning. A pleading in law means a written statement of a cause pro and con., and “special pleaders” are persons who have been called to the bar, but do not speak as advocates. They advise on evidence, draw up affidavits, state the merits and demerits of a cause, and so on. After a time most special pleaders go to the bar, and many get advanced to the bench.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Special Pleading from Infoplease: