It is just like English sayings and phrases, there are Japanese sayings and phrases as well. It is called [kotowaza ことわざ].
鬼に金棒 Oni ni Kanabo
Oni – a deman
Kanabo – a metal rod
make one even stronger
If a stone demon gets a metal rod it make him even stronger!
Person who is powerful already become even much stronger by adding much power.
My brother joins the strongest baseball team in the city. What’s more, a famous coach will be coming next week, well, it is Oni ni kanabo!