Lewis & Short

simplus, a, um.

  1. I. Adj. [kindr. with simplex], simple; as adj. only post-class.: mors, Prud. στεφ. 10, 878.
    More freq.,
  2. II. Substt.
    1. A. simplum, i, n., that which is single or simple, the simple sum or number (opp. to the double): duplum simplum, Cic. Top. 11, 49: simplum solvere, Plaut. Poen. 5, 6, 25 (opp. duplum, id. ib. 5, 6, 14): si simplum imperetur, Liv. 29, 15; Dig. 49, 14, 5.
    2. B. simpla, ae, f. (sc. pecunia), the simple purchase-money, Varr. R. R. 2, 10, 5; Dig. 21, 2, 60.