not to have the foggiest idea

I haven't the foggiest idea

Ich habe nicht die leiseste Ahnung

He did not have the foggiest idea what had been talked behind his back.

Er hatte nicht die leiseste Ahnung, was war hinter seinem Rücken gesprochen.

He did not have the foggiest idea what they were talking behind his back.

Er hatte nicht die leiseste Ahnung, was sie hinter seinem Rücken reden.

He does not have the foggiest idea what they were talking behind his back.

Er hat nicht die leiseste Ahnung, was sie hinter seinem Rücken reden.