A phrase used during a gossip session when the subject of gossip is seen approaching, or caught within ear shot of your conversation to cover the real content of the story.
Sheri - "Did you see the jeans Amy was wearing yesterday?"
Chris - "Yeah, seriously, a 9 inch zipper...its not 1985 anymore."
(Amy walks in room)
Sheri - "Oh, but that was yesterday...hey Amy, what's up?"