noun
complete understanding; knowing everything about something
Used when something is fully understood, leaving nothing unclear. Often appears in phrases like 丸わかりだ or 丸わかりになる.
この説明書を読めば、使い方が丸わかりだ。
If you read this manual, you'll understand how to use it completely.
彼の表情を見れば、気持ちは丸わかりだ。
You can tell exactly how he feels just by looking at his expression.
Compound of 丸 (まる, 'whole, complete') and わかり (the noun form of わかる, 'to understand'). The exact origin is uncertain, but it follows a common pattern of prefixing 丸 to emphasize completeness.