Naughty; mischievous; unruly—used as a quasi-adjective to describe boisterous or unruly behavior, often of children.
あの子はやんちゃで先生を困らせることがある。
That child is mischievous and sometimes troubles the teacher.
昔はやんちゃだったが、今は落ち着いた。
He was wild when he was younger, but he's calm now.
やんちゃな子猫が家中を走り回っている。
A mischievous kitten is running around the house.
A rascal; a naughty kid—used as a noun for someone who behaves mischievously.
近所のやんちゃが道路で遊んでいるので注意した。
I warned the local rascals who were playing in the street.
やんちゃ坊主と呼ばれるほど元気だ。
He's so lively he's called a little rascal.
Delinquency; misconduct—used colloquially to refer to straying into wrongdoing or antisocial behavior.
若い頃のやんちゃが原因で後悔している友人がいる。
I have a friend who regrets the delinquent behavior he displayed when he was young.
やんちゃに走らないよう、家庭でしっかり躾ける必要がある。
Children need proper upbringing at home so they don't fall into delinquency.
Bold; audacious; flamboyant—used for outfits or behavior that are daring or showy.
彼は最近、やんちゃなファッションを好むようになった。
Lately he has started to prefer bold, flashy fashion.
そのデザインは少しやんちゃすぎるかもしれないが、面白い。
The design might be a bit too flashy, but it's interesting.