This is a short form of adjective, accusative case.

It answers the question какову мысль? скорбну.

It is a rare word in modern Russian, but valid when such adjective takes the place of verb, or always valid in case of poetry.

You can look them up using ruscorpora if you use brev grammatical qualifier.

This is the short form of the adjective in the accusative.

It answers the question какову мысль? скорбну.

It is a rare word in modern Russian but valid when this adjective takes the place of a verb, and always valid in poetry.

You can look them up using ruscorpora if you use the grammatical qualifier brev.