Originally a noun (and still a noun in some isolated uses), the adjective dismal comes into English, like many of our words, with the Normans, a compound formed from the Old French phrase dis mal, which in turn is from the Latin dies mali or “bad days.” The noun dismal, meaning bad or unlucky days, appears in English c. 1300. For example, around 1369 Chaucer writes in The Book of the Duchess, lines 1206–07:
I trowe hyt was in the dismalle,
That was the .x. woundes of Egipte.
(I believe it was in the dismal,
that was the ten wounds of Egypt.)
The dismal, also called the Egyptian days because they were first calculated by Egyptian astrologers, consisted of two days per month on which it was unlucky to start a journey or begin a venture. In the Middle Ages, the days became associated with plagues of Egypt described in Exodus, hence the Chaucer quote.
By the fifteenth century the association with the Latin dies, “days,” had been sufficiently forgotten that people started referring to them with the redundant dismal days, as in Lydgate’s Siege of Thebes, written c. 1421, line 2893:
He hath pronounced [...] his Cleer conceyte [...] her dysemol daies and her fatal houres.
(He has pronounced [...] his clear understanding [...] her dismal days and her fatal hours.)
By the sixteenth century, dismal was being used as an adjective meaning unlucky or disastrous. By the seventeenth century it was being used to mean dark, gloomy, or cheerless.
Sources: “dismal, n. and adj.” Oxford English Dictionary, 2nd edition, 1989; “dismal, n. and adj.” Middle English Dictionary, 2001.
Copyright 1997-2014, by David Wilton