joist (n.) Look up joist at
"timbers supporting a floor, etc.," early 14c. gist, giste, from Old French giste "beam supporting a bridge" (Modern French gîte), noun use of fem. past participle of gesir "to lie," from Latin iacēre "to lie, rest," related (via the notion of "to be thrown") to iacere "to throw" (see jet (v.)). The notion is of a wooden beam on which boards "lie down." The modern English vowel is a corruption.