kimochi ii 気持ちいい

In Japanese, kimochi ii 気持ちいい means literally "of good feeling," or less literally "pleasant." It's an expression used when something "feels good," like rain, for example, but grammatically it's classified as an i-adjective.

The word is also spelled kimochi ii 気持ち良い, and kimichi-yoi 気持ちよい is a synonymous variant.

Manga: Kids on the Slope, Sakamichi no Apollon 坂道のアポロン (Chapter 2)

Grammar

The word kimochi ii 気持ちいい means that something "feels good." Literally, it's the word for "feeling," kimochi 気持ち, plus "good," ii いい.

The antonym, "feels bad," is kimochi warui 気持ち悪い, since warui 悪い, "bad," is the antonym of "good," ii いい.

Note, however, that kimochi ii isn't a verb, "to feel good," it's adjective, "that feels good." Grammatically, kimochi ii is closer to "pleasant" in English, but in practice it translates to "feels good" most of the time.

Furthermore, from an English perspective, it's weird for an adjective to be alone in a sentence. Normally you don't say "fast," you say "it's fast." This is called a "dummy it." There's no "it" in Japanese, which is why kimochi ii is said alone in Japanese.

kimochi ii! 気持ちいい！
[It's] good-feeling!
[It's] pleasant!
[It feels good]!

Usage

For reference, some examples of things that are kimochi ii 気持ちいい.

Here with kimochi ii in the predicative:

ame ga kimochi ii
雨が気持ちいい
The rain is pleasant.
The rain [feels good].
massaaji ga kimochi ii
マッサージが気持ちいい
The message is pleasant.
The massage [feels good].
kaze ga kimochi ii
風が気持ちいい
The wind is pleasant.
The wind [feels good].

And here with kimochi ii in the attributive:

kimochi ii uta 気持ちいい歌
A pleasant song.
A pleasant music.
kimochi ii oto 気持ちいい音
A pleasant sound.

Sometimes it's connected after the te-form of another adjective. For example:

tsumetai 冷たい
Cold.
tsumetakute kimochi ii
冷たくて気持ちいい
Cold and pleasant.
- Something that's cold to the touch, and touching it feels good.

Note that something that "tastes good" is oishii 美味しい, not kimochi ii.

Conjugation

Since kimochi ii is an i-adjective, it can be conjugated like an i-adjective.

Kimochi yokatta 気持ちよかった

The word kimochi yokatta 気持ちよかった means "felt good." In the past.

Manga: Kids on the Slope, Sakamichi no Apollon 坂道のアポロン (Chapter 2)

karada ni ataru ame wa
体に当たる雨は
The rain [that] hit [my] body.
omotteta yori
思ってたより
More than [I] thought.
zutto
ずっと
(intensifier.)
kimochi yokatta-n-da
気持ちよかったんだ
Felt good.
Stringed together:
- The rain [that] hit [my] body
  felt much better
  than [I] had thought.

Kimochi yokunai 気持ちよくない

The word kimochi yokunai 気持ちよくない means "doesn't feel good." It's negative.

And kimochi yokunakatta 気持ちよくなかった means "didn't feel good." It's past negative.

Kimochi yoku 気持ちよく

The word kimochi yoku 気持ちよく is the adverbial form of kimochi ii. It's used before a verb, when you're doing something that feels good to do.

Manga: Doraemon ドラえもん (Chapter 10, おせじ口べに)

e wo kaiteiru no.
絵をかいているの。
Are [you] drawing a picture?
un,
うん、
Yes,
hisashiburi ni
久しぶりに
For the first time in a while
kimochi yoku
kaketa.
気持ちよくかけた。
[I] was able to draw feeling good.
- i.e. for the first time in a while, he was able to enjoy himself drawing.
- kaketa 描けた
  Was able to draw.
  (past form of.)
- kakeru 描ける
  To be able to draw.
  (potential form of.)
- kaku 描く
  To draw.

Kimochi yosa 気持ちよさ

The word kimochi yosa 気持ちよさ is the sa-form of kimochi ii, which is the "-ness form." It means "good-feeling-ness," or "how good it feels."

kono kimochi yosa
この気持ちよさ
This good-feeling-ness. (that I'm feeling.)
How good it feels [now].

Kimochi yosasou 気持ちよさそう

The word kimochi yosasou 気持ちよさそう is the word kimochi ii plus the sou suffix, which means "seems," so "seeming it's pleasant," or "seeming it feels good."

kimochi yosasou
気持ちよさそう
[That] looks like [it] feels good.
kimochi yosasou na kao
気持ちよさそうな顔
A face [that] looks like [it] feels good. (e.g. your expression under a kimochi ii rain.)