Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
merto, 1, v. freq. a. [mergo], a collat. form of merso, to immerse, overwhelm (anteclass.): mertat pro mersat dicebant, Paul. ex Fest. p. 124 Müll.; cf. Quint. 1, 4, 14: quos hic non mertet metus, Att. ap. Non. 138, 33; 138, 2.