After his duel with Oliver Queen during the end of the League civil war, Malcolm Merlyn's left hand was cut off. Days later, he got himself a new, cybernetic hand to be able to fight and do basic tasks that would be impossible to complete for one-handed individuals. The first known time Malcolm used the hand was when he stole the Idol from Team Arrow, and he has been wearing it ever since.
After the Legion of Doom used the Spear of Destiny to alter reality, they also used the Spear to give Malcolm his left hand back. Because of that, the cybernetic hand has no use. It's unknown what they did with it. However, after the Legends undo what the Legion had done, Malcolm's left hand was never returned to him, and he still uses his cybernetic hand.