Phrase meanings

I have been unable to find the origin of the phrase "close enough for government work." While it is now derogatory and suggestive of sloppy work, I was under the impression it originated with a meaning just the opposite due to stringent specifications for government construction contracts. ...

Explain the expression--"you can't prove a negative," and how one would use it in a sentence.

Hi all, I noticed the following phrase on some newspaper: "in-house vendor". I'm not an English, and I have look for these words in the dictionary. The dictionaries tell me that "in-house" means make by ourselves Vendor means someone who selling something But, what is the whole phrase...