“Feckless,” on the other hand, comes not from Latin (at least not directly) but from dialects spoken in Scotland and northern England. It was first recorded in the late 1500s and means—listen for the echo—ineffective.
We know what you’re thinking. Is there a word “feck,” to which “less” was added? The answer is yes!
“Feck” is in fact a Scottish shortening of “effect,” Chambers says. And the ancestor of “effect” is the Latin verb efficere, meaning to work out or bring about. The Latin word is a compound of ex (out) and facere (to make or do).
The noun “feck” was first recorded in the late 1400s and means, in the words of the OED, “operative value, efficacy, efficiency” and hence also “vigour, energy.” It’s still used in parts of Britain today.
Originally, the OED says, “feckless” was used to describe things (not people) that were considered “valueless, futile, feeble.”
Later, it was used chiefly to describe people believed to be “lacking vigour, energy, or capacity; weak, helpless; (now more usually) irresponsible, shiftless.”
So “feckless,” too, has shifted gears somewhat. These days, it often describes not just incapacity or inability but moral weakness.
http://www.grammarphobia.com/blog/2011/10/feckless.html
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