In a manner that is permitted or acceptable; in a way that follows rules or is allowed.
From 'allowable' plus the adverbial suffix '-ly,' which comes from Old English '-lice' meaning 'like' or 'in the manner of.' This is one of English's most productive word-formation patterns.
The '-ly' ending is so common we barely notice it, but it's one of the most reliable ways to spot an adverb in English—and it's been working the same way for over 1,000 years, making it one of the language's most stable features.
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