Feckless is an adjective and frequently treated as an orphan negative. No really, it's from a complete group of derivations. The root noun is feck meaning "vigor, power, control of oneself," the adverb is "feckfully" while the adjective is feckful "vigorous, powerful, effective." You can still find them in Oxford English Dictionary, and thus are available for use by those with the feck to do so.
Pronounced 'fek-lis, it means to be ineffective, irresponsible, or incompetent, as in, "Oh, sharper than a serpent's tooth is a feckless child."
For some time feckless was used mainly in the Scottish dialect. For example from Sir Walter Scott wrote "Ye'll no do for Steenie, lass--a feckless thing like you's no fit to mainteen a man." It arose circa 1585 as a derivative of feck from effect and English-less without. Thomas Carlyle popularized it with his writings, considered the Irish to be a feckless and lazy people in his not so humble opinion and lost friends as a result. His wife Charlotte Carlyle came to think of him as a feckless drunk.
Now how did the Scots go about changing effect into feck? Some linguistic scholars think it may have come from the confusion of where to draw the line in the phrase theffect, as "the effect" is commonly pronounced. By becoming a clipped form of the phrase, they reason that aphesis may have occurred, which simply means that the Scots dropped the initial unaccented vowel. Pretty cool word.
Sources:
The Maven's Word of the Day:
http://www.randomhouse.com/
yourDictionary.com:
http://www.yourdictionary.com/