Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
per-sīdo, sēdi, sessum, 3, v. n., to sink or settle down, to penetrate anywhere (poet.): pestilitas fruges persidit in ipsas, Lucr. 6, 1126; 1, 307: ubi frigidus imber Altius ad vivum persedit, Verg. G. 3, 442.