unbeknown
1. [ adverb ] without someone's knowledge
Synonyms: unbeknownst
Examples:

"unbeknownst to me, she made all the arrangements"

2. [ adjective ] (usually used with `to') occurring or existing without the knowledge of
Synonyms: unbeknownst
Examples:

"a crisis unbeknown to me" "she had been ill for months, unbeknownst to the family"

Related terms: unknown
Similar spelling:   unbeknownst