adverb
with all one's might; wholeheartedly
Colloquial variant of 思いっきり, used adverbially to mean doing something with full effort or without holding back.
See also: 思いっきり
思いっきし走ったら、すごく疲れた。
I ran as hard as I could and got really tired.
今日は思いっきし楽しもう！
Let's have a blast today!
noun
Colloquial noun use, often in the phrase 思いっきしをつける (to make up one's mind, to resign oneself). Equivalent to 思いっきり as a noun.
See also: 思いっきり
思いっきしをつけて、仕事を辞めることにした。
I made up my mind and decided to quit my job.
思いっきし is a colloquial contraction of 思いっきり, used in casual speech. 思いっきり is the standard form and more widely accepted in both spoken and written Japanese.
Colloquial contraction of 思いっきり (omoikkiri), where the final り (ri) is replaced by し (shi), a common sound shift in casual speech.