noun, na-adjective, noun which may take the genitive case particle 'no'
Describes a disorderly mixture of various things, often in a playful or casual tone. Can be used as a noun or adjectivally with な or の.
引き出しの中がまぜこぜで、何がどこにあるかわからない。
The drawer is a jumble, and I can't tell what's where.
いろんな色のビーズがまぜこぜになった。
Beads of various colors got all mixed up.
ごちゃごちゃ is more common and can be used as an adverb or する-verb, while まぜこぜ is a noun/adjective emphasizing a mixed-up state.
混ざる is a verb meaning 'to be mixed', while まぜこぜ is a noun/adjective describing the resulting mixed state.
Reduplicative formation from 混ぜ (maze, 'mix') with emphatic repetition, likely of mimetic origin. The exact derivation is uncertain.