The Collaborative International Dictionary of English v.0.48:

Non assumpsit \Non` as*sump"sit\ [L., he did not undertake.]
   (Law)
   The general plea or denial in an action of assumpsit.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

NON ASSUMPSIT, pleading. The general issue in trespass on the case, in the
species of assumpsit. Its form is, "And the said C D, by E F, his attorney,
comes and defends the wrong and injury, when, &c., and says, that he did not
undertake or promise in manner and form as the said A B, hath above
complained. And of this he puts himself upon the country."
     2. Under this plea almost every matter may be given in evidence, on the
ground, it is said, that as the action is founded on the contract, and the
injury is the non, performance of it, evidence which disaffirms the
obligation of the contract, at the time when the action was commenced, goes
to the gist of the action. Gilb. C. P. 6 5; Salk. 27 9; 2 Str. 738; 1 B. &
P. 481. Vide 12 Vin. Ab. 189; Com Dig. Pleader, 2 G 1.