adverb
A somewhat literary or formal adverb meaning that something happens without prior expectation or by coincidence. Often used in written Japanese or polite speech.
はしなくも彼と再会することになった。
Unexpectedly, I ended up meeting him again.
はしなくもその秘密を知ってしまった。
By chance, I came to know that secret.
偶然 is a more common and neutral word for 'by chance' or 'accidentally'. はしなくも carries a slightly more literary or formal tone and emphasizes the unexpectedness of the event.
思いがけず also means 'unexpectedly', but is more conversational and less formal than はしなくも.
Derived from the adjective 端ない (はしない, 'unexpected') with the adverbial suffix く and the particle も. The exact historical development is uncertain, but the word has been used in classical Japanese.