Meaning

Completely mad. This is now commonly understood to mean crazy, although the original meaning is unclear and may have meant annoyed.

Origin

Mercury used to be used in the making of hats. This was known to have affected the nervous systems of hatters, causing them to tremble and appear insane. A neurotoxicologist correspondent informs me that “Mercury exposure can cause aggressiveness, mood swings, and anti-social behavior.”

Raining cats and dogs

Meaning

Raining very heavily.

One Possible Origin (There are many theories.)

The phrase is supposed to have originated in England in the 17th century when city streets were filthy and heavy rain would occasionally carry along dead animals.

As happy as a clam

Meaning

Very happy and content.

Origin

An early version is ‘as happy as a clam at high water’. Clams are free from the attentions of predators at high tide, so perhaps that’s a reason to consider them happy then.

The phrase originated in the US and possibly before 1834. In 1848 the Southern Literary Messenger – Richmond, Virginia expressed the opinion that the phrase “is familiar to everyone”.

That should probably be “mad as a hatter” and it’s a reference to the fact cheap top hats used to be made with a mercury, a potent neurotoxin. Mad here refers not to insanity but to anger.

> raining cats and dogs

It means raining heavily.

> happy as a clam

It refers to the line formed by the clam’s top and bottom shells coming together to form what looks like a smile.

As to how they came into existence… well someone thought them up and other people liked the phrases enough to use them in their own speech.

Check out the Memes and Memetics for more depth on the creation of these kinds of phrases.