Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
arra, arrabo, arralis, v. arrha, arrhabo, arrhalis.
† 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).