Dave Wilton at Wordorigins.org has a post about a great etymology that I’d forgotten if I ever knew it:
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. [...] 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. [...] 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 [...] 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.
(Visit the link for supporting quotes.) The American Heritage Dictionary adds that in the South Atlantic states of the U.S., “a swamp or marsh can be called a pocosin or a dismal, the second term illustrated in the name of the Dismal Swamp on the border of North Carolina and Virginia. The word pocosin possibly comes from Virginia Algonquian.”