v.tr. (3rd sing. present -does; past -did; past part. -done) 1 a unfasten or untie (a coat, button, parcel, etc.). b unfasten the clothing of (a person). 2 annul, cancel (cannot undo the past). 3 ruin the prospects, reputation, or morals of.
[ OE undon (as UN-(2), DO(1)) ]