from
Bouvier's Law Dictionary, Revised 6th Ed (1856)
AFFIRMATIVE PREGNANT, Pleading. An affirmative allegation, implying some
negative, in favor of the adverse party, for example, if to an action of
assumpsit, which is barred by the act of limitations of six years, the
defendant pleads that be did not undertake &c. within ten years; a
replication that he did undertake, &c. within ten years, would be an
affirmative pregnant; since it would impliedly admit that the defendant had
not promised within six years. As no proper issue could be tendered upon
such plea the plaintiff should, for that reason, demur to it. Gould, Pl. c.
6 29, 37; Steph. Pl. 381; Lawes, Civ. Pl. 113; Bac. Ab. Pleas, N 6.