the word "unbeknownst" is a relatively modern english formation, built from older english elements. it combines the negative prefix "un-" with the archaic past participle "knownst," which itself derives from the old english verb "cnāwan," meaning "to know." the "be-" prefix, also from old english, intensifies or surrounds the action, giving a sense of something being "about" or "around" in terms of knowledge. thus, the literal construction "not + thoroughly + known" evolved into the modern adjectival sense of something happening without a specific person's awareness.

Structure: un (not) + be (about) + knownst (known) un- (old english, a prefix meaning "not"); be- (old english, a prefix with a sense of "about, around, thoroughly"); knownst (archaic english, past participle form of "know").