The difference between へとへと and くたくた

Question

I am studying Japanese オノマトペ and I came across two that are very similar. The definitions I found online, and the examples sentences in the book don't align, so I am confused about the 使い方。

On this website, they say that へとへと describes a more exhausted condition than くたくた. They describe へとへと as "so tired that you can't get off the floor" and くたくた as "Tired, but if you rest for a while, you will be good again", but, in the JLPT book that I am studying from, the example sentences make it sound like the difference is between physical exhaustion and mental exhaustion.

• くたくた：

  六時間も山道を登ってきたので、くたくただ。

• へとへと：

  六時間連続で試験を受けて、もうへとへとだ。

So what actually is the difference?

Answer 1

According to the corpus provided by The National Institute for Japanese Language and Linguistics, contrary to the examples in the original question, there seem to be plenty of examples of both くたくた referring to mental exhaustion and へとへと referring to physical exhaustion.

くたくた mental exhaustion:

• 周辺の人も屈託に巻き込まれてくたくたになる。
• そのうち、旦那さんのほうもうすうすガンだと気づいていながら、奥さんを気づかって知らないふりをする。 そのあげく、双方で余計な疲れ方をして、くたくたになっちゃうというケース。
• 寝る前の飲み物を控えても、近頃では毎朝おねしょとなり、毎日の寝具の洗濯でクタクタになってきて、ノイローゼになりそうです。

へとへと physical exhaustion:

• 不忍池を歩き尽くして、へとへとになって澤の屋に戻ってきた。
• それでも私たちはへとへとになりながら、ようやく頂上に這い上がった。
• 二日続きの徒歩旅行のため、波止浜・大井・菊間まで来ると、ヘトヘトになって動かなくなる者も出て、船で帰る者もいた。

Based on looking through most of the examples though, it indeed appears that へとへと is used more frequently to indicate mental exhaustion like in the examples in the original question. Maybe it's because that's considered the more severe form of exhaustion.

It should also be noted that くたくた can refer to things other than exhaustion. For example in cooking:

• 物の形がくずれるほどによく煮るさま。また、そのときの音を表す語。ぐたぐた。ぐつぐつ。 「 －と煮込む」

• 物が、細かくなってしまうさま。ずたずた。こなごな。 「背骨を打ち切りて、－となしつ／宇治拾遺 12」

• からだがひどく疲れて力が抜けたさま。 「一日歩いて－だ」 ② 服などの形がくずれたさま。 「雨にぬれて－になった洋服」

Source

Answer 2

I discussed this with a native speaker and we came to the following conclusion — this is only intended as a point of reference and may be "just another theory".

へとへと is usually used for yourself and can refer to both mental and physical exhaustion, but seems to be mainly subjective.

くたくた seems to refer to a change of state. For example, it has been mentioned that くたくた can also be used in cooking and りんごをくたくたになるまで煮てください means to simmer the apple until it loses its "crunch/snap" and becomes soft.

Note that くたくた seems to be derived(?) from the verb くたびれる and the translations of being "worn out" or "bashed up" seem to aptly describe the "change of state" we conjecture.

When expressing that you yourself are exhausted, you might use へとへと to say you have no more "energy", whereas you might use くたくた to refer to a change of state, such as sore muscles or sore joints (e.g. from walking all day). That said you might also use くたくた for the change of state from "having energy" to "having no energy", so there would be some overlap — conversely, へとへと could be used to emphasize that you feel exhausted, because of sore muscles/joints, but I would still say that へとへと is more about your subjective feeling than about something tangible.

Answer 3

I think both Ringil's and Earthliŋ's answers above are good. I offer this as an additional perspective.

Looking at where words come from, and what other words they are related to and associated with, helps place an individual term within the web of vocabulary. As such, we might find hints at the shades of meaning in the derivations and related terms of both words.

へとへと

Appears to be derivationally onomatopoeia, based on the sound of someone breathing hard. Its sound-shape is also quite close to 辺【へた】 or 端【へた】, alternative reading へち, with a basic meaning of "edge", potentially alluding to the limit of one's energy or abilities.

くたくた

Related to 草臥【くたび】れる "to become exhausted; to wear out", with the underlying root apparently kut-, as also seen in verbs 腐【くた】す "to cause to wear out, break down, or rot", and 朽【く】ちる "to rot and deform; to break down".