Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
rārĭtūdo, ĭnis, f. [rarus], looseness (rare form for raritas): rete a raritudine (dictum), Varr. L. L. 5, § 130 Müll.: mediocri raritudine terra, Col. Arb. 3, 7.