without the knowledge of; in a way that is unknown to a particular person.
From 'unknown' with an archaic past participle ending '-st' or '-t,' similar to 'whilst' or 'betwixt.' It's an older English form that persists as an idiom, technically redundant since 'unknown' alone carries the same meaning.
Grammarians argue that 'unbeknownst' is actually redundant because 'unknown' already means the same thing—you don't need to say 'unbeknownst to me' when 'unknown to me' works fine! Yet everyone keeps using the longer version anyway.
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