Webster's English Dictionary

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joist

joist \'jo.ist\ n [ME giste, fr. MF, fr. (assumed) VL jacitum, fr. L 
   jace-re] to lie - more at ADJACENT 1: any of the small timbers or metal 
   beams ranged parallel from wall to wal l in a building to support the floor 
   or ceiling 2: a stud or scantling about 3 by 4 inches in section