Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
fĭdĕprōmissor, ōris, m. [fidepromitto], jurid. t. t., a surety, bail: pro eo, qui promittit, solent alii obligari: quorum alios sponsores, alios fidepromissores, alios fidejussores appellamus. Sponsor ita interrogatur: IDEM DARE SPONDES? fidepro missor: IDEM FIDEPROMITTIS? fidejussor ita: IDEM FIDE TVA ESSE JVBES? etc., Gai. Inst. 3, § 115:
In another orthogr.: fidei promissor, πιστικελευστης, Gloss. Philox.