Meaning/Usage: Something really important. Often times used with "no" to express that the event is not as important as they think.
Explanation: This phrase is also used sarcastically. For example, if someone says "I did 30 push-ups this morning." Someone else can respond, "Big deal. I did 50 push-ups!"
Passing my driver's test was no big deal.
He thinks he is such a big deal.
It's no big deal if we don't go tonight.
A.
I think I might have lost that screwdriver you let me borrow last week.