頓珍漢

先生（せんせい）に急（きゅう）に当（あ）てられて、トンチンカンな答（こた）えをしてしまった。

I gave an answer that was way out in left field when the teacher caught me off-guard and called on me.

私（わたし）の祖父（そふ）は耳（みみ）が遠（とお）いので、よくトンチンカンな返事（へんじ）をする。

My grandfather, being hard of hearing, often makes an irrelevant answer.