1833, vulgar formation from unbeknown (1630s). No clear reason for the -st, but since 19c. this has become the dominant form.
unbeknownst to me, she made all the arrangements
she had been ill for months, unbeknownst to the family
unbefitting
unbefriended
unbegotten
unbegun
unbeknown
unbeknownst
unbelief
unbelievable
unbeliever
unbend
unbeseeming
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