I know many of you cannot resist to be harsh, therefore, I have added a list of harsh words.
As much use as a dead fox- A (harsh) insult meaning the recipient is useless
Bees in your brain - An exclamation meaning a confused or a cat not making sense (i.e. You've got bees in your brain!)
Beetle-brain - A playful insult similar to mouse-brain
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."
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
Dormouse - An expression or exclamation used against a cat who sleeps a lot.
Drypaw- A cat that dislikes getting wet, usually used in RiverClan.
like LionClan/TigerClan - An expression stating that a cat does something very fiercely or well
Fishface, fishfur or fish-breath - An insult used against RiverClan warriors.
Fishbrain - Same as mouse-brain and Frog-brain except it's often used in RiverClan.
Fish-dung - Same as mouse-dung Same as mouse-brain except it's often used in RiverClan.
Flea-pelt - A mean insult saying that a cat has lots of fleas and that they aren't fit to do anything.
Frog-brain - Same as mouse-brain except it's often used in RiverClan.
Frog-dung - Same as fox-dung except it's often used in RiverClan.
Snake-hearted - Similar to the term "Fox-hearted," generally used by RiverClan. Used in Crookedstar's Promise.
Fox-dung - A (harsh) insult comparing the recipient to fox feces. Also used as an exclamation (as in That's fox-dung!)
Fox-hearted - An insult meaning cruel, cold-hearted, or evil (as in fox-hearted Twolegs).
Furball - A (friendly yet harsh) insult. Severity of the insult is almost always decided by tone.
Great StarClan! - An exclamation used to signify extreme surprise or anger. Similar to Oh my gosh! in usage and to the fact that they both have mention of a higher force
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 in The Sight[70]
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
Make dirt -to use the bathroom[72]
May StarClan light your path/Banish all the fleas from your nest - A friendly term used to tell others they wish them well
Mouse-brain - A (friendly yet harsh) insult. Severity of the insult is almost always decided by tone.
Mouse-dung - A (somewhat harsh) insult comparing the recipient to mouse feces. Also used as an exclamation (as in Mousedung! That can't be right!). Somewhat like "Darn it!"
Mouse-hearted - An insult that describes one who is cowardly.
Once a kittypet, always a kittypet! - A harsh phrase used to describe a cat with kittypet roots
(Only) StarClan knows (what) - Sometimes used to answer a question that is impossible to answer. Similar to "Only God knows what," "Only God knows," and "God knows."
Scaredy-mouse! - A similar expression to scaredy-cat, but in cat terms
Sorry catches no prey or Sorry fills no bellies - Means cats can be sorry, but that will not undo what happened
StarClan's kits! – An exclamation of surprise or disbelief. Sometimes used as an exclamation if a cat is hurt.
What in StarClan's name?- A exclamation similar to "What on earth?" or "What in the world?"
Furball - A friendly or harsh insult for a cat (as in stupid furball, bossy furball or lazy furball)
Thistles and Thorns - Used to describe bad luck (as in: Thistles and thorns! I blew it!) similar to "Darn it" or "mouse-dung"
Thundercat - Used to describe a ThunderClan cat.
You fight/hunt like a kittypet! - An expression used to insult or tease a cat who fights/hunts poorly.
Who made dirt in his fresh kill?- A phrase to describe a cranky or suddenly moody cat.
Would've made mousemeat out of you! - To badly injure, rip to shreds.
Resources-
http://warriors.wikia.com/wiki/Clan_Terminology