#1 – for all I know
This expression means “according to the information I currently have” but implies uncertainty, like you don’t actually have much information.
I’m not eating those mushrooms you found in the woods. They could be poisonous for all I know.
#2 – know your stuff
If someone knows his/her stuff, it means the person is very knowledgeable and skilled in that area.
This carpenter really knows his stuff because he has 30 years of experience.
#3 – know the drill
To know the drill means to be familiar with a standard procedure or what is supposed to be done.
Nicole has organized the past three conferences, so she knows the drill.
#4 – know where you stand
To know where you stand means to be aware of your current position/situation.
I know where I stand and I’m not authorized to make that decision without consulting the director.
#5 – it takes one to know one
This phrase is used to insult someone by saying that they are the same as the other person who they are criticizing/accusing/complaining about.
Bob says that John is a hypocrite, but it takes one to know one.
(= Bob is also a hypocrite)
#6 – you don’t know the half of it
This expression means someone only knows about a small part of a situation, but there is a much bigger issue that they are not aware of.
“I heard you’ve been a bit busy with work these days.”
“Oh, you don’t know the half of it! Not only have I been working 60-hour weeks, but also my kids got sick and the house needed some urgent repairs.”
#7 – not know what hit (someone)
If someone doesn’t know what hit them, it means they are very surprised and shocked by an unexpected event that has a big negative impact on them.
When my wife left me for no reason, I didn’t know what hit me. I’d been under the impression that our relationship was fine.
#8 – know no bounds
This expression describes something that is or seems to be without limit.
She’s always coming up with new ideas. Her creativity knows no bounds.
#9 – What does he/she know?
This question is used to say that someone does NOT know anything about a particular topic.
I wish my brother would stop giving me parenting advice. What does he know? He doesn’t even have kids.
#10 – Who knows?
This expression means that nobody probably knows the answer.
“How long is this flight going to be delayed?”
“Who knows? I guess it depends on how long it keeps raining.”