Definition of "improbably"

[…] and two enormous Scottish poems, the Buik of Alexander, which has been improbably ascribed to Barbour, and Sir Gilbert Hay's Buik of Alexander the Conquerour; one nearly complete Prose Life of Alexander and fragments of four others; a stanzaic translation of the Fuerres de Gadres which survives only in a fragment, the Romance of Cassamus, and three separate translations of the Secreta Secretorum.

2012, William Matthews, The Tragedy of Arthur‎, University of California Press, page 68