guiltlessly comes from guiltless (Old English gyltleas) + 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 manner free from guilt or blame", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as guiltless, guilt, and guilty. Its semantic neighborhood includes innocently, blamelessly, and faultlessly, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English gylt.