"covering over part of a ship," mid-15c., perhaps a shortening of Middle Low German verdeck (or a related North Sea Germanic word), a nautical word, from ver- "fore" + decken "to cover, put under roof," from Proto-Germanic *thackjam (related to thatch, q.v.).
Sense extended early in English from "covering" to "platform of a ship." "Pack of cards" is 1590s, perhaps because they were stacked like decks of a ship. Deck chair (1884) so called because they were used on ocean liners. Tape deck (1949) is in reference to the flat surface of old reel-to-reel tape recorders.