加減 is a useful but tricky word. By itself, it indicates balance, a state, or moderation.
But... that's not how it's usually used.
First, let's break down the kanji:
Kanji Parts: 力 (power) + 口 (mouth)
Meanings: add; addition; increase; join; include
Kun: くわ.える、 くわ.わる
Kanji Parts: 氵 (water) + 戈 (weapon) + 一 (one) 口 (mouth)
Meanings: dwindle; decrease; reduce; decline; curtail; get hungry
Kun: へ.る、 へ.らす
So, if you add a little 加 and subtract a little 減, you get Goldilocks. Just right.
But as with most useful things, it isn't quite that easy. While it can and does have the meaning of "just right" it can also mean the opposite.
Here are three meanings for いい加減:
1) Negative: irresponsible; careless; half-baked
That person is careless.
2) (Usually) Negative: very; considerably; quite [this indicates that a certain degree has been reached and the speaker thinks it should stop]
I've just about had enough of him.
3) Positive: reasonable; moderate; just right
The bath water is just right (temperature).
How can it mean "just right" and "not right (sloppy)"?!
The original meaning is "just right" but the most common usage is
This means "Cut it out!" or "That's enough!"
The speaker wants the listener to be like this and therefore the いい加減 part has a positive meaning even though it is used in a negative situation. But because this is in a negative situation, the いい加減 has taken on a mostly negative connotation in modern Japanese.
This is how we get meanings such as:
That person is careless.
Even though, you might look at the kanji and assume this means "This person is just right" it means quite the opposite. This person is sloppy or careless
Quick and dirty rule: Unless the situation is indicating otherwise--for example the bath water is just the right temperature or a bag is just the right size for its contents--think of いい加減 as a negative: "that's enough" "sloppy" "cut it out" "get serious" "quit playing around"
Other 加減 words:
measuring by feel; knack; skill - this is using your hand to adjust, and therefore it also has the meaning of going easy on someone or adapting to the situation.
Do I need to go easy on you?
(For example, when you are about to play a game with someone.)
spoonful; a dose (of medicine); adjusting cooking with seasoning; a feel for doing something; a knack; making adjustments based on experience and knowledge
This dish is very tasty, with just the right amount of seasoning.
extent of (one's) foolishness
I am very ashamed of my stupidity.
And lastly, a less idiomatic but very common example is.
water temperature, especially the bath
The bath water is just right.