Noun
(law, archaic) The party to a legal dispute who first raised a complaint in a court.
(law, archaic) The first party named in a contract, generally the party agreeing to perform a duty thereunder.
Source: en.wiktionary.orgThe first part of the party of the first part shall be known in this contract as the first part of the party of the first part shall be known in this contract-- Look, why should we quarrel about a thing like this? We'll take it right out, eh? Groucho Marx