Lewis & Short

arrha, ae, f., and arrhăbo (also without aspiration arra and arrăbo), ōnis, m. (the latter form ante-class.; cf. Gell. 17, 2, 21; in Cic. the word is never used), = ἀρραβών [from the Heb. [??] from [??], to give security], the money given to ratify a contract, earnest-money, purchase-money, a pledge, an earnest (arrha is a part of the purchase-money, while pignus is a pledge to be restored when the contract, for security of which it is given, Las been performed, Isid. Orig. 5, 25).

  1. I. Lit.: arraboni has dedit quadraginta minas, Plaut. Most. 3, 1, 115; id. Rud. prol. 46; id. Poen. 5, 6, 22: Ea relicta huic arrabonist pro illo argento, * Ter. Heaut. 3, 3, 42: tantus arrabo, Quadrig. ap. Gell. 17, 2, 20 (i. e. sexcentos obsides, Gell.): dederis mihi arrabonem, Vulg. Gen. 38, 17: pro arrabone dari, ib. ib. 38, 18.
    Jestingly shortened into rabo: rabonem habeto, mecum ut hanc noctem sies, Plaut. Truc. 3, 2, 20 sq.
  2. II. Trop.: arrabo amoris, Plaut. Mil. 4, 1, 11; Dig. 18, 1, 35; Plin. 33, 1, 6, § 28; and so ironically: mortis arra, money given to physicians, Plin. 29, 1, 8, § 21.