also: えいやっ
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.
Katakana emphatic variant with a glottal stop.
Katakana spelling, common in manga or emphatic contexts.
The origin is uncertain; it is a sound-symbolic interjection that naturally expresses sudden effort or determination.