Nah, but i've heard of it
A lie your friend tells to save face when they don’t know what you're talking about but think they do
'Nah, but I've heard of it' when you told them about a new movie and they had no idea what it was about.
'Nah, but I've heard of it' when you mentioned a new slang word and they just nodded like they understood.
'Nah, but I've heard of it' when you asked if they knew the name of the president and they said they did, but they didn't.