This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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v. To make or become weak or weaker.

v. To make weaker.

v. To become weaker.

v. To make weak; to lessen the strength of; to deprive of strength; to debilitate; to enfeeble; to enervate.

v. To reduce in quality, strength, or spirit.

verb-intransitive. To become weak or weaker; to lose strength, spirit, or determination; to become less positive or resolute.

To become weak or weaker: as, he weakens from day to day.

To make weak or weaker; lesson or reduce the strength, power, ability, influence, or quality of: as, to weaken the body or the mind; to weaken a solution or infusion by dilution; to weaken the force of an argument.

v. become weaker

v. destroy property or hinder normal operations

v. reduce the level or intensity or size or scope of

v. lessen in force or effect

v. lessen the strength of

"But by the late 18th and early 19th centuries, that empire was beginning to weaken from a series of financially ruinous European wars, the cost of supplying and fortifying their outposts in the Americas, piracy and other factors."

weakened    weakening    weakens   

minify    lessen    decrease   

backslide    dilute    adulterate   

debilitate    enfeeble    enervate    languish    flag    fail    decline   

Mohican    Pekin    beacon    deacon    pekin   

strengthen    loosen    tighten    demoralization    lower    soften    widen    deterioration    devaluation    retrogression