noun
Used to refer to the entirety of something, often in the phrase 合切袋 (がっさいぶくろ) meaning a bag containing all one's belongings. The standalone use is somewhat literary or old-fashioned.
合切袋一つで旅に出た。
He set off on a journey with just one bag containing all his belongings.
彼は財産を合切失った。
He lost his entire fortune.
全部 is the common, everyday word for 'all' or 'everything'. 合切 is rare and carries a nuance of 'everything one has' or 'the whole lot', often used in set phrases.
一切 means 'all' or 'everything' but is more common in negative sentences (一切〜ない). 合切 is not restricted to negative contexts and is less common.
The exact derivation is uncertain. The kanji 合 (together) and 切 (cut/limit) may suggest 'all together' or 'the whole lot', but the word is not transparently derived from these meanings.