The headword is the aorist passive participle, masculine nominative singular, of the verb parata/ssw (I place side by side, marshal, draw up in order of battle); see generally LSJ s.v. It has evidently been extracted from somewhere. One possibility is Aeschines 3.35, where it is used in a transferred sense (of the opponent Ktesiphon making common political cause with Demosthenes). But more probably (see next note) the source is the earliest instance of this form: Plato, Republic 556D (web address 1); Socrates reflects on the reaction a sinewy, sunburned pauper might have upon being stationed in battle-formation alongside a pale, plump rich man.
[1] The headword is identically glossed in Timaeus; Platonic Lexicon (pi999a20) and Photius' Lexicon (pi315 Theodoridis).