What a nice word. It can mean counter clockwise, or to the left of something, or contrary to the motion of the sun, or just going in the opposite direction from the one you were expecting. According to the OED the earliest recorded use of the term is 1513, when it appears in Gavin Douglas’s translation of the Aeneid of Virgil. Upon seeing a ghost his character admits, ’widdersyns start my hair’ (‘my hair stood on end’).