It comes from the fact that a male chicken, a rooster, rules over the females, or hens. The place where birds sit is called a roost, so if you "rule the roost," you dominate the rest of your group. They do what you tell them, and defer to your decisions. Sometimes you see this used humorously to mean that in a marriage, the wife "rules the roost" instead of the husband.
A roost is a place where birds perch -- the dominant bird rules this area and bosses the other birds around. A rooster is a male chicken. This is a humorous way of saying that the man may rule the house, but his partner (often a woman) rules over him.
A difrence between workers and bosses, possibly a strike or work to rule.
You mean Valoo? He's at the top of the mountain.
It is not an idiom, it means your nose is itching.
It's not really an idiom. It means "what are you thinking about."
RFP is not an idiom. It's an abbreviation.
Roost is a noun (a roost) and a verb (to roost).
"Sieve" is not an idiom. See the related link.
It's not an idiom. It means the tip of your nostril.
idiom means expression like a page in a book
The idiom "a slap on the wrist" means receiving a mild or lenient punishment for a wrongdoing. It implies that the consequence is not severe or harsh.
This is not an idiom. It is a measurement. $100,000 is how you write it in numbers.