Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sigla, ōrum, n. [contr. from sigilla], signs of abbreviation, abbreviations (late Lat.): sigla in libris ponere, Just. Ep. ad Antecess. § 8: leges per siglorum obscuritates conscribere, Cod. Just. 1, 17, 2, § 22.