hiddenly comes from hidden, the past participle of hide (Old English hȳdan 'to conceal'), + the adverbial suffix -ly. Morphologically, the ending is part of the etymology: the suffix is attached to the older base named in the history, producing a later English derivative. In present English, the adverb sense is "in a concealed or secret manner", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as hidden, hide, and hiddenness. Its semantic neighborhood includes secretly, covertly, stealthily, and clandestinely, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English hȳdan and Proto-Germanic *hūdijaną.