Drool

In manga and anime, sometimes characters are drawn drooling. This can happen for various reasons:

They're sleeping, dozing off, or bored.
They're stupid.
They're hungry for food, or lusting for something, someone.

Jibril ジブリール, example of character drooling, yodare 涎.

Character: Jibril ジブリール
Anime: No Game No Life, ノーゲーム・ノーライフ (Episode 6)

In Japanese
Rendering
Examples
- Sleeping
  - Sleepy
- Stupid
- Hungry
- Greed
- Lewd

In Japanese

The word for "drool" in Japanese is yodare よだれ, also spelled yodare ヨダレ and yodare 涎. This isn't a jouyou kanji 常用漢字, so the word is typically spelled without kanji.

To say "to drool" in Japanese, the noun for "drool" is combined with the ergative verb pair tareru 垂れる, tarasu 垂らす, meaning "to drip."

yodare ga tareru
よだれが垂れる
Drool drips.
yodare wo tarasu
よだれを垂らす
To drip drool. To drool.

Manga: Hinamatsuri ヒナまつり (Chapter 3, 授業参観は突然に)

Context: Mishima Hitomi 三嶋瞳 finds Hina ヒナ asleep and drooling.
n
ん
Hmm.
yodare tareteru yo
よだれたれてるよ
Drool is dripping.
- (i-nuki-kotoba 居抜き言葉.)
- tarete-iru
  垂れている
  To be dripping.
goshi'
ゴシッ
*scrubbing clean*
n
ん
Hmm.
aa, seifuku de shicha
ああ　制服でしちゃ
Aah, [don't do it] using [your] uniform.
- __ shicha dame
  〇〇しちゃ駄目
  Doing __ is bad.
  You shouldn't do __.
  Don't do __.

Some vocabulary:

tsuba
唾
Spit.
- tsuba wo tsukeru
  唾を付ける
  To attach spit. (literally.)
  In the sense of having already put something in your mouth, so other people shouldn't eat it: to call dibs, to say something is yours and other people shouldn't take it.
da-eki
唾液
Saliva.

Rendering

In some cases, drool is rendered in manga and anime simplified, by merging the shape of the drool with the shape of the mouth, so it looks like the lower lip is dripping. In this case, the drool becomes the same color of lower part of the mouth. For example:

Character: Izumi Konata 泉こなた
Anime: Lucky☆Star, らき☆すた (Episode 4)

In extreme cases, the "drool" will drips lower than the character's jaw and it will make no sense physically speaking.

Examples

For reference, some examples of anime characters drooling:

Sleeping

Character: Kagamihara Nadeshiko 各務原なでしこ
Anime: Yuru Camp△, ゆるキャン△ (Episode 1)

negao
寝顔
Sleeping face.

Sleepy

Hirasawa Yui 平沢唯, example of character drooling, yodare ヨダレ.

Character: Hirasawa Yui 平沢唯
Anime: K-On!, Keion! けいおん！ (Episode 3)

Stupid

Manga: Saiki Kusuo no Psi-Nan, 斉木楠雄のΨ難 (Chapter 2, 最低Ψ悪！？燃堂力)

Context: Saiki Kusuo 斉木楠雄 explains why he can't figure out what Nendou Riki 燃堂力 is thinking.
riyuu wa
sugu wakatta
理由はすぐ分かった
The reason [I] understood immediately.
paaan
ぱーーーん
*ta da!* (sound effect used when something is presented, in this case, Nendou, the character.)
wahihi ahii
わひひ　あひー
*idiotic snickering*
kono otoko
baka
nano de aru
この男バカなのである
This man is an idiot.