making the strong even stronger; adding power to power
Idiomatic expression used when someone already strong or capable gains an additional advantage, making them nearly unstoppable. Literally 'giving an iron club to an ogre'.
He was already fast, and with the new shoes he's unstoppable.
あのチームにエースが復帰したら、まさに鬼に金棒だね。
If the ace returns to that team, they'll be truly invincible.