Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Algĭdum, v. Algidus.
2. Algĭdus, i, m., a high snow-capped mountain, and the forest upon it, south-east of Rome, between Tusculum and Velitrœ, now Monte Compatri, Liv. 26, 9, 12; Plin. 18, 13, 34, § 130: gelidus, Hor. C. 1, 21, 6: nivalis, id. ib. 3, 23, 9; id. C. S. 69.
Hence,