To unlash is to unfasten something that has been secured with rope or cord.
Sailors unlash equipment lashed to the deck; campers unlash gear from a pack frame.
The word simply reverses lash in its binding sense: We unlashed the canoe from the roof rack.
1. release — unfasten, Release from attachment; untie (Undo a fastening); unbind (To release from bonds); loosen (Reduce tension or grip); detach · free · disengage · unhook · unstrap · unclasp · unshackle · unbolt · uncouple · unpin
2. nautical — rope, Thick twisted fibers; line (Continuous mark or queue); cord (Flexible twisted strand); rigging (Ropes controlling ship sails); mooring · deck · cargo · knot · fasten · secure · sail · mast · boom · cleat