Meaning: To be poor, to have difficulty supporting yourself.
Example: Most people who marry young start out living hand to mouth.
Origin: During the Great Depression and other times of economic scarcity, people often did not know when or where the next meal was coming from. In such a case, when you get something in your hand that can be eaten, it goes into the mouth immediately: ergo "hand to mouth."
Example: Don't let the cat out of the bag about Susan's Christmas present.
Origin: At medieval markets, unscrupulous traders would display a pig for sale. However, the pig was always given to the customer in a bag, with strict instructions not to open the bag until they were some way away. The trader would hand the customer a bag containing something that wriggled, and it was only later that the buyer would find he'd been conned when he opened the bag to reveal that it contained a cat, not a pig. Therefore, "letting the cat out of the bag" revealed the secret of the con trick.
Meaning: You must keep this secret or there will be a price paid.
Example: Do not discuss any of our plans outside this boardroom, remember loose lips sink ships.
Origin: Popularized by US military propaganda during World War II. The phrase was meant as a reminder that classified information was never to be discussed with anyone without proper clearance and a need to know.
Specifically the phrase means that disclosing a military secret to the enemy could result in large loses, such as the sinking of an entire ship.
Meaning: A person who is out of control, unpredictable, who may do damage.
Example: The typical Engineer is too honest for his own good, they can be like a loose cannon around customers.
Origin: On sailing ships that had cannons, it was important that they be secured. Cannons are very heavy, and a loose cannon on a ship's deck in a rough sea could be thrown about in an unpredictable fashion, causing a lot of damage.
More than just needing to be lashed down during normal travel, cannons needed to be secured during use, or else the recoil would send the cannon on its way causing injury or damage.
It's Old English form is actually "hl��fd�ge," which was changed to "lafdi" or "ladi" during the Middle English years. Either way, it's original meaning was "loaf-kneader."