Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
pī̆gresco, ĕre, v. inch. n. [pigreo], to become slow, sluggish: Nilus pigrescit, Plin. 18, 18, 47, § 168: possit pigrescere Cyllenius, Mart. Cap. 1, § 38; Ambros. Virg. 17, 110; id. in Luc. 10, § 23.