思いっ切り
おもいっきり · omoikkiri · JLPT N2 · adverb
very
思い (feeling) + 切る (cut through) = cut through with all your feeling, fully
切 — 七 (phonetic) + 刀 (knife radical)
切 is a semantic-phonetic compound: 刀 'knife' gives the meaning of cutting, while 七 supplies the sound. The original sense was to slice or cut with a blade. From there it extended figuratively to 'close to' (as in pressed against) and 'urgent/earnest', as in 切实.