na-adjective, noun which may take the genitive case particle 'no', noun
brand new; fresh; completely unused
Describes something in a pristine, untouched state, often with a sense of purity or newness. Can be used for objects, clothes, notebooks, or even abstract things like a fresh start.
まっさらなノートを開く。
I open a brand new notebook.
彼はまっさらのスーツを着て面接に行った。
He went to the interview in a brand new suit.
新品 means 'new article' or 'unused item', focusing on the fact that it hasn't been used. まっさら emphasizes the pristine, untouched state, often with a nuance of freshness.
真新しい is an i-adjective meaning 'brand new', similar to まっさら, but まっさら can also imply a clean slate or pure state, not just newness.
Derived from 真っ (ma', intensifier) + さら (sara, 'new' or 'fresh'). The exact origin of さら is uncertain, but it may be related to 更 (further, anew) or 新 (new).