Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sub-tristis, e, adj., somewhat sad or sorrowful (ante- and post-class.): subtristis visus est mihi, Ter. And. 2, 6, 16; Hier. Ep. 107, 9.
Comp.: cera subtristior, Hier. Ep. 60, 1.