na-adjective
A direct loan from English, used in Japanese to describe actions, ideas, or people as lacking good sense. Often carries a slightly literary or emphatic tone compared to native words like 愚か (oroka) or 馬鹿げた (bakageta).
そんなフーリッシュな考えは捨てたほうがいい。
You should give up such a foolish idea.
彼の行動はまったくフーリッシュだった。
His behavior was completely foolish.