DONE FOR (SLANG)

Meaning and connotations:

a. Dead
b. Doomed
c. About to fail
d. Worn out (dead tired)

Examples:

a. This car is just done for, don't you think?

b. This situation puts us on a tight spot. We are DONE FOR

Sour grapes

 a. Something that one cannot have and so disparages as if it were never desirable.

b. If you say that something someone says is sour grapes, you mean that they said it because they are jealous 

c. Something you say when someone pretends not to be jealous

Examples:

I don't think it's such a great job - and that's not just sour grapes because I didn't get it.

Are his criticisms justified, or is this a case of sour grapes from a less successful artist?

A boy fancied a girl,but another boy took her.He pretended not to care,but his friends said 'sour grapes

The expression "WEAK AT THE KNEES"

If someone goes weak at the knees, they feel as if they might fall down because they have a sudden strong emotion about something or someone: "The very thought of jumping out of an aircraft with a parachute made him go weak at the knees". He was so gorgeous, I felt weak at the knees every time he spoke to me.