StarClan must have shared their wisdom with this article, because it was previously featured on the front page.
This is a collection of terms used by the Clancats in their speech. Just as we humans have sayings, the Clan cats have idioms, too, although theirs relate to things found in the world of the Clan cats.
Fresh-kill – A recently killed prey animal caught for the purpose of consumption. Fresh-kill is hunted by warriors and apprentices, and sometimes elders, and placed on the fresh-kill pile or brought to the elders. Elders, kits, queens, and sick cats eat first, then warriors and apprentices. Extras stay in the fresh-kill pile.
Fourtrees - A place located in the forest territory, where the territory's corners would meet, in which four oaks stood and the Clans would gather in peace every full moon.
Gathering – The monthly meeting of all the Clans when the moon is full, at either Fourtrees or the Island. There is a truce on this night, and no fighting is allowed.
Greenleaf Twolegplace – A place frequented by Twolegs during summer, such as a resort or camping spot near the lake.
Sunup - Dawn. However, Dawn is still used for example in the phrase "Dawn Patrol."
Fox-length - About the length of a fox; approximately a yard. (80 cm)
Kittenstep or kitstep - About the length of kit's step; approximately an inch or a half inch. (1.25-2.5 cm)
Tail-length - About the length of a cat's tail; approximately a foot. (30 cm)
Rabbit hop or rabbit length - About a foot and a half away. (45 cm)
Mouse-length - About two or three inches. (5-7.5 cm)
Pawstep - About the length of a grown cat's step, roughly six inches. (15 cm)
Tree-length - About the length of a tree; around 40 to 50 feet. (15 m)
Insults, exclamations, & phrases
As easy as swallowing a minnow - A phrase used to indicate easiness of the task.
As much use as a dead fox - A (harsh) insult, meaning the recipient is useless.
Bees in your brain - An exclamation meaning confusion or a cat not making sense. (i.e., You've got bees in your brain!)
Buzzardfood- Alternative of crowfood, used before the start of the Clans. Used in The Sun Trail.
Chickfeed- Alternative of crowfood, used before the start of the Clans. Used in The Sun Trail.
Cleanpaw- An (scornful) insult used for someone who sticks to the rules closely. 
Crowfood - A (harsh) insult comparing the recipient to rotting prey animals (as in I'll turn you into crowfood!), or suggesting that they eat it (as in crowfood-eater).
(You're) crazier than a fox in a fit! - An insult used when a cat (or cats) are acting crazy. Another variation uses "madder" instead of "crazier." Lionblaze uses this to describe Ashfur.
Doesn't matter a whisker/Doesn't catch so much as a whisker - An exclamation meaning the cat does not care, (i.e. it didn't matter a whisker that she didn't share their beliefs) similar to I don't give a mousetail. Or saying that a patrol didn't catch anything.
Furball - A (friendly, yet harsh) insult. Severity of the insult is almost always decided by tone.
Fuzz-brain - A (friendly, but harsh) insult. Severity of the insult is almost always decided by tone. Used in The Sun Trail by Gray Wing when Jagged Peak leaves the mountain cave.
Great StarClan! - An exclamation used to signify extreme surprise or anger. Similar to Oh my god! in usage and to the fact that they both have mention of a higher force.
Hare-dung - Used as an exclamation, similar to "mouse-dung". Somewhat like "Darn it!" Used in The Sun Trail. 
When hedgehogs will fly - An exclamation style statement that shows disbelief that a certain event will likely occur (as in That will happen when hedgehogs fly), used in many variations. Somewhat like the human phrase, When pigs fly!
How in Silverpelt? - An exclamation used meaning "How in the world?" Used inThe Sight.
I don't give a mousetail/They wouldn't give a mousetail - An exclamation meaning that the cat does not care. Another variation is I would ____ for a couple of mousetails meaning that they are willing to do the inserted action for no real reward. Can also mean that a cat would be stingy enough not to share a mousetail.
I'd have shredded you into mousedust! - to gravely injure, rip to shreds. Similar to mousemeat. Used by Yellowfang in Into the Wild while intimidating Firepaw.
It's better to scare away a mouse than welcome a badger - An old nursery saying that means that it's better to scare away a friendly cat than welcome a not-so-friendly cat. Used by Cloudtail in Dark River when an apprentice almost scared Mothwing away.
Make dirt - to use the bathroom, or the dirtplace in warrior terms.
May StarClan light your path/Banish all the fleas from your nest - A friendly term used to tell others they wish them well.
May the Sun warm your back and the fish leap into your paws- A blessing used in RiverClan used to wish good dreams or, in some instances, to say farewell to a dead cat.
May you find good hunting, swift running, and shelter when you sleep- An ancient farewell used by all the Clans, to say goodbye to loved ones on their way to StarClan.
Apprentice - A cat at least six moons old, and training to be a warrior (or a medicine cat.)
Warrior - A mature cat who has completed his or her warrior training. A she-cat is considered a warrior when she is not currently expecting or nursing a tom's kits.
Mentor - A cat who is currently training an apprentice.
Queen - A she-cat with kits to take care of or who is pregnant with kits, or stays in the nursery helping other queens even though they have no kits. (i.e. Daisy, Ferncloud)
Elder - Cats who cannot hunt or fight anymore, sometimes due to injury (Stonepelt), loss of sight (Longtail), sickness (Heavystep), or are just too old to do their warrior duties. They are treated with great respect in the Clan. The apprentices in the Clan take fresh kill to them, pick out their fleas, and change their bedding.
Deputy - A warrior who is second in command to the leader, helping the leader in their tasks and replacing them after their death or resignment. They organize patrols and make reports to the leader, and are known to be frequently given apprentices.
Leader - A cat who controls the entire Clan. A leader is granted nine lives by StarClan so they "can be first in every battle and last to take a piece of fresh-kill" basically giving them extra time to serve and lead their Clan.