The big (number)-o phrase informal
An age that ends in a zero, especially as marked by a landmark birthday
My grandpa turned the big ten-o yesterday, and my family organized a great birthday party for him.
My ambition is to become rich before reaching the big three-o.
Used to indicate that the age at which a person's consent to sexual relations is legitimate.
Be nearly a particular age
Another individual is far older than the individual in question
Said when a person is in a romantic or sexual relationship with another younger person
Look older than a certain age
The origin of this phrase is not clear.
1. Doing something extremely well
2. Behaving in an extreme or remarkable manner
1. Holly was in rare form during her speech, so she got the first prize in the competition last night.
2. Corrine: I'm going to buy Christmas presents for everybody. Do you want to go with me?
Laura: Oh...My kids are in rare form today, so I cannot go with you. Sorry!