“Coincidence? I think not!”
Two of your coworkers are out sick on the same day. You think they are secretly dating each other. You mention to your friend that they are both absent from work, and that you think they aren't really sick.
Coincidence? I think not!
In normal speech, we don’t usually say “I think not,” because it’s an old-fashioned form of English. (We usually say “I don’t think so.”) Using this older phrase sounds more dramatic. You can use it to express a strong feeling against something, with a joking tone.
Share a room with a stranger? I think not!
Nine dollars for a smoothie? I think not!
This phrase sounds very serious and dramatic. In the past, you might hear it used in movies and TV shows, especially about crime and mysteries. People like to use this phrase, often as a joke. We don’t usually use such serious, dramatic language in everyday conversation.
My cookie disappeared, and you have crumbs all over your lap. Coincidence? I think not!
You and Jenny both left the party early. Coincidence? I think not!