The Collaborative International Dictionary of English v.0.48:

Lower \Low"er\, v. i. [imp. & p. p. Lowered; p. pr. & vb. n.
   Lowering.] [OE. lowren, luren; cf. D. loeren, LG. luren. G.
   lauern to lurk, to be on the watch, and E. leer, lurk.]
   1. To be dark, gloomy, and threatening, as clouds; to be
      covered with dark and threatening clouds, as the sky; to
      show threatening signs of approach, as a tempest.
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            All the clouds that lowered upon our house. --Shak.
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   2. To frown; to look sullen.
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            But sullen discontent sat lowering on her face.
                                                  --Dryden.
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The Collaborative International Dictionary of English v.0.48:

Lower \Low"er\, v. t. [imp. & p. p. Lowered; p. pr. & vb. n.
   Lowering.] [From Low, a.]
   1. To let descend by its own weight, as something suspended;
      to let down; as, to lower a bucket into a well; to lower a
      sail or a boat; sometimes, to pull down; as, to lower a
      flag.
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            Lowered softly with a threefold cord of love
            Down to a silent grave.               --Tennyson.
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   2. To reduce the height of; as, to lower a fence or wall; to
      lower a chimney or turret.
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   3. To depress as to direction; as, to lower the aim of a gun;
      to make less elevated as to object; as, to lower one's
      ambition, aspirations, or hopes.
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   4. To reduce the degree, intensity, strength, etc., of; as,
      to lower the temperature of anything; to lower one's
      vitality; to lower distilled liquors.
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   5. To bring down; to humble; as, to lower one's pride.
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   6. To reduce in value, amount, etc.; as, to lower the price
      of goods, the rate of interest, etc.
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The Collaborative International Dictionary of English v.0.48:

Lowering \Low"er*ing\, a.
   Dark and threatening; gloomy; sullen; as, lowering clouds or
   sky.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lowering
    adj 1: darkened by clouds; "a heavy sky" [syn: heavy,
           lowering, sullen, threatening]
    n 1: the act of causing to become less
    2: the act of causing something to move to a lower level [syn:
       lowering, letting down]

Moby Thesaurus II by Grady Ward, 1.0:

220 Moby Thesaurus words for "lowering":
   Acheronian, Acherontic, Cimmerian, Stygian, abatement, about to be,
   abridgment, abusive, alleviation, already in sight, apocalyptic,
   approaching, at hand, attenuation, bad, baleful, baneful, beetle,
   beetle-browed, beetling, black, black-browed, bludgeoning,
   blustering, bodeful, boding, break, brewing, browbeating,
   bulldozing, bullying, cadence, catenary, cheapening, close,
   close at hand, clouded, cloudy, coming, comminatory, concavity,
   contraction, cut, dampening, damping, dark, dark and gloomy,
   de-escalation, debasement, decline, decrease, decrement,
   decrescence, decurrence, deduction, deepening, deflation,
   degradation, dejected, demotion, denunciatory, depreciation,
   depression, descent, detrusion, devaluation, digging, diminishment,
   diminution, dire, dive, doomful, dour, dreary, drilling, droop,
   ducking, dull, dumpish, dying, dying off, evil, evil-starred,
   excavation, extenuation, fade-out, fateful, fear-inspiring,
   foreboding, forthcoming, frowning, funereal, future, gathering,
   gloom, gloomful, gloominess, glooming, gloomy, glowering, glum,
   going to happen, grim, grum, hauling down, heavy, hectoring,
   hollowness, ill, ill-boding, ill-fated, ill-lighted, ill-lit,
   ill-omened, ill-starred, immediate, imminent, impendent, impending,
   in danger imminent, in prospect, in reserve, in store,
   in the cards, in the offing, in the wind, in view, inauspicious,
   incumbent, instant, intimidating, jutting, languishment, lapse,
   lessening, letup, looming, lower, lurking, markdown, melancholy,
   menacing, minacious, minatory, miniaturization, mining, mitigation,
   moodish, moody, mopey, moping, mopish, morose, mumpish, near,
   near at hand, nearing, nose dive, nubilous, of evil portent,
   ominous, on the horizon, overcast, overclouded, overhanging,
   overhung, pending, plummet, plummeting, plunge, portending,
   portentous, preparing, price cut, price fall, price reduction,
   probing, projecting, reduction, relaxation, sag, sagging,
   scaling down, scowling, simplicity, sinister, sinkage, sinking,
   slash, slump, somber, somberness, sombrous, sombrousness, stormy,
   submergence, subsidence, subtraction, sulky, sullen,
   superincumbent, surly, swag, terroristic, terrorizing,
   that will be, threatening, threatful, thrusting under, to come,
   tunneling, unfavorable, unfortunate, unlucky, unpromising,
   unpropitious, untoward, upcoming, waiting, weakening