Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
trĕmesco (trĕmisco), ĕre, v. n. and a. inch. [tremo], to begin to shake or tremble, to shake, quake, or tremble for fear; to quake or tremble at a thing (poet.): plaustri concussa tremescunt Tecta viam propter, Lucr. 6, 548: tonitruque tremescunt Ardua terrarum, Verg. A. 5, 694: jubeo tremescere montes, Ov. M. 7, 205: latitans omnemque tremescens Ad strepitum, id. ib. 14, 214.
With acc.: sonitumque pedum vocemque tremesco, Verg. A. 3, 648: Phrygia arma, id. ib. 11, 403.
With object-clause: telum instare tremescit, Verg. A. 12, 916.
With rel.-clause: quercum nutantem nemus et mons ipse tremescit, Quā tellure cadat, Stat. Th. 9, 535.