[高中]請問their "whitest"意思

[ADDIE (....)]

大家好, 本人想請問一個字:

原文:
White clothes, or "whites" as they are often called,
are usually washed in warm water,
and often with a bit of bleach to keep them their whitest.

想請問最後一個字"whitest"的詞性與意思

我的猜測：
詞性：是否為形容詞最高級當名詞用??
意思：是否為"最白的狀態"?

因為查過多本字典與文法書，但沒找到類似用法
先謝謝解答^_^

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◆ From: 122.116.71.83
※ 編輯: ADDIE           來自: 122.116.71.83        (01/10 15:24)
推 outofdejavu:It is of informal usage.
→ outofdejavu:And "their whitest" is a fixed expression.
推 outofdejavu:You'll have a clear idea if you google "the
→ outofdejavu:whitest" yourself.
→ ADDIE:Thank you.^_^