Feckless describes people or things that are weak or ineffective.

// The agency’s response to the dramatic increase in air pollution was well-intentioned but ultimately feckless.

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"The players streamed down Columbus Avenue, serenading passersby with the Tilted Axes theme song; a pedestrian stopped and stared. When the Axes crossed Sixty-sixth Street, traffic momentarily isolated one bass player from the rest of the band, like a feckless baby elephant stranded on the veldt." — Henry Alford, The New Yorker, 22 July 2024

A feckless person is lacking in feck. And what, you may ask, is feck? In Scots—our source of feckless—feck means "majority" or "effect." The term is ultimately an alteration of the Middle English effect. So something without feck is without effect, i.e., ineffective. In the past, feckful (meaning "efficient, effective," "sturdy," or "powerful") made an occasional appearance, but in this case, the weak has outlived the strong: feckless is a commonly used English word, but feckful has proven, well, feckless.