英語のことわざ

Jack of all trades, master of none.

This proverb is a warning, If you try to do too many different things, you will never be very good at one thing. "Jack" used to be a common English name and so it means "a person." "Trade" here means something like "job," and "all" means "many." So, a person who has many different jobs will never be a master at one of them.