Common American Phrases in Everyday Contexts (249 page)

You couldn't (do that)
!
AND
You wouldn't (do that)
!
an indication of disbelief that someone might do something.
B
ILL
: I'm going to run away from home! J
ANE
: You couldn't!
B
ILL
: I get so mad at my brother, I could just strangle him. T
OM
: You couldn't do that!

You do the math
.
The conclusion is clear and simple, and you can figure it out by yourself.; It's a monetary decision and an easy one at that.
A new one is $400 and I can get a reconditioned one for $150. You do the math
.
It's as big a car as we can afford. You do the math!

You doing okay?
Go to (Are you) doing okay?

You don't expect me to believe that
.
Go to You can't expect me to believe that.

You don't know the half of it
.
You really don't know how bad it is.; You might think that what you have heard is bad, but you do not know the whole story.
M
ARY
: They say you've been having a bad time at home. S
ALLY
: You don't know the half of it
.
S
ALLY
: The company has no cash, they are losing orders right and left, and the comptroller is cooking the books. M
ARY
: Sounds bad. S
ALLY
: You don't know the half of it.

You don't know where it's been
.
It may be dirty, so do not touch it or put it in your mouth, because you do not know where it has been and what kind of dirt it has picked up. (Most often said to children.)
M
OTHER
: Don't put that money in your mouth. You don't know where it's been. B
ILL
: Okay
.
F
ATHER
: Take that stick out of your mouth. You don't know where it's been. B
OB
: It's been on the ground.

You don't say. 1
.
a general response to something that someone has said. (Expresses a little polite surprise or interest, but not disbelief.)
B
ILL
: I'm starting work on a new job next Monday. B
OB
: You don't say
.
S
ALLY
: The Jones boys are keeping a pet snake. A
LICE
: You don't say
.
2
.
You have just said something that everybody already knows.
B
ILL
: I think I'm beginning to put on a little weight. J
ANE
: You don't say
.
J
OHN
: My goodness, prices are getting high. S
UE
: You don't say.