The word 'split-up' is a nominalization of the phrasal verb 'to split up'. The core verb 'split' entered English from Middle Dutch splitten, meaning to cleave, which is likely of imitative origin, echoing the sound of something tearing apart. The particle 'up', from Old English, often adds a sense of completion or finality to a verb, as seen in 'break up' or 'finish up'. Thus, the literal action of "splitting completely" evolved into the modern abstract noun specifically denoting the definitive end or division of a union, whether personal or corporate. This compound exemplifies how English creates specific nouns from common verbal phrases to describe complex social and commercial events.

Structure: split (to divide) + up (completive particle) split: Of Germanic origin (Middle Dutch splitten), meaning to cleave or rend. In this compound, it functions as the core verb root. up: Of Germanic origin (Old English up), meaning upward or completely. In this phrasal verb nominalization, it functions as an adverbial particle emphasizing completion or finality.