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][1] if you use the grammatical qualifier `brev`.


  [1]: https://ruscorpora.ru/results?search=CskBEqUBCqIBChMKCWRpc2FtYm1vZBIGCgRtYWluChcKB2Rpc3Rtb2QSDAoKd2l0aF96ZXJvcxJyChkKA2xleBISChDRgdC60L7RgNCx0L3Ri9C5CgoKBGZvcm0SAgoACg8KBWdyYW1tEgYKBGJyZXYKCQoDc2VtEgIKAAoVCgdzZW0tbW9kEgoKCHNlbXxzZW14CgkKA3N5bhICCgAKCwoFZmxhZ3MSAgoAKhgKCAgAEAoYMiAKEAUgAEAFagQwLjk1eAAyAggBOgEBMAE=