Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
lassesco, ĕre, v. inch. n. [lassus], to become tired, to grow weary (post-Aug.): ne lassescat fortuna, Plin. 7, 40, 41, § 130; 14, 2, 4, § 33: victo lassescere visu, Prud. in Symm. 2, 101: lassescere sub lorica, Hier. Ep. 22, 39: non bibet aquam et lassescet, Vulg. Isa. 44, 12.