un ‧ be ‧ known /ˌʌnbɪˈnəʊn $ -ˈnoʊn/ BrE AmE ( also un‧be‧knownst /-ˈnəʊnst $ -ˈnoʊnst/ written ) adverb [sentence adverb]
unbeknown to somebody without that person knowing about it:
Unbeknown to him, his wife had been trying to phone him all morning.