I heard it was on - A fancy way of lying, like you're saying something that's so

A fancy way of lying, like you're saying something that's so obviously false it's almost insulting.

I heard it was on, and I'm telling you it was a complete lie.

I heard it was on, and I'm not even gonna bother arguing with you.

I heard it was on, and I'm just here to say it was a total fake.