Discard, reject, as in He cast off his clothes and jumped in the pool. This term was already used figuratively in Miles Coverdale's translation of the Bible (1535): "Thy mother ... that hath cast off her housebonds and her children" (Ezekiel 16:45).
Let go, set loose, as in He cast off the line and the boat drifted from the dock. [Second half of 1600s]
In knitting, to finish the last row of stitches, that is, take the stitches off the needle and form a selvage. For example, Your sweater is finished; I just have to cast off. [Late 1800s] Also see cast on, def. 1.