also: えいやっ
interjection
An exclamation used when making a sudden effort, lifting something heavy, or psyching oneself up. Covers both physical exertion and mental determination.
「えいや！」と叫んで、重い箱を持ち上げた。
With a shout of "Heave!", I lifted the heavy box.
えいやっ！と気合を入れて、試験に臨んだ。
I psyched myself up with an "Ugh!" and faced the exam.
よいしょ is more commonly used for physical effort like lifting or standing up, while えいや can also express mental determination or a sudden burst of energy.
どっこいしょ is a longer, more drawn-out exclamation often used by older people when sitting down or getting up, whereas えいや is shorter and sharper.
The origin is uncertain; it is a sound-symbolic interjection that naturally expresses sudden effort or determination.