“I was completely oblivious!”
Two of your friends have been secretly dating each other. Finally they tell you about it, and you're really surprised because you didn't suspect it. You say this.
I was completely oblivious!
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Being "oblivious" to something means that you don't notice it, or don't know that it's happening. It sounds a little negative but not too bad.
The word "oblivious" often appears together with "completely":
Most people are completely oblivious to all of the ways that they're being manipulated by advertising and the media.
If you want to include the topic, use "oblivious to __":
I was completely oblivious to the fact that they were dating.
When you're high, you become oblivious to everything that's going on around you.
"Oblivious" is an advanced word, so not everyone uses it. A more basic word is "unaware".