まったく心当たりが無いを英語でなんと言うか？

not have the faintest idea, not have the foggiest idea, not have the slightest idea のような言い方になります。3:53 I know you're not going to believe this, but I haven't the slightest idea where those came from.

MacGyver Every Time She Smiles Penny Parker
https://www.youtube.com/watch?v=PkFFSp4CE_Q  1:13 I haven't the foggiest idea. まったく見当が付かないね。
Super Suction
https://www.youtube.com/watch?v=MiEjj_ynQtg  7:11 we haven't the foggiest idea what they're even talking about.
Top 10 Lies We All Tell
https://www.youtube.com/watch?v=8IAIJQpQuSM  0:22 you're the first person to notice. no one from our office had the slightest idea. It's called a panic room.
Panic Room - Trailer

練習問題解答
a-6516 don't seem to have the faintest idea why
b-0984 haven't the faintest idea