It's on the tip of my tongue.
1. It's on the tip of my tongue, let me see.
2. His address is on the tip of my tongue, wait a minute.
3. The question she wanted to ask was on the tip of her tongue.
A: Did you see the new employee in our company?
B: Yeah. Do you know her name?
A: It's on the tip of my tongue but I just can't remember.
B: Well, let's just ask the manager?
A: I've got a better idea. Let's just ask her instead.