1. threatening(s = adj.all) baleful, forbidding, menacing, minacious, minatory, ominous, sinister - threatening or foreshadowing evil or tragic developments; "a baleful look"; "forbidding thunderclouds"; "his tone became menacing"; "ominous rumblings of discontent"; "sinister storm clouds"; "a sinister smile"; "his threatening behavior"; "ugly black clouds"; "the situation became ugly"
2. threatening(s = adj.all) heavy, lowering, sullen - darkened by clouds; "a heavy sky"
N danger, peril, insecurity, jeopardy, risk, hazard, venture, precariousness, slipperiness, instability, defenselessness, exposure, vulnerability, vulnerable point, heel of Achilles, forlorn hope, leap in the dark, road to ruin, faciles descensus Averni, hairbreadth escape, cause for alarm, source of danger, rock ahead, breakers ahead, storm brewing, clouds in the horizon, clouds gathering, warning, alarm, apprehension, in danger, endangered, fraught with danger, dangerous, hazardous, perilous, parlous, periculous, unsafe, unprotected &c (safe, protect), insecure, untrustworthy, built upon, sand, on a sandy basis, wildcat, defenseless, fenceless, guardless, harborless, unshielded, vulnerable, expugnable, exposed, open to, aux abois, at bay, on the wrong side of the wall, on a lee shore, on the rocks, at stake, in question, precarious, critical, ticklish, slippery, slippy, hanging by a thread, with a halter round one's neck, between the hammer and the anvil, between Scylla and Charybdis, between a rock and a hard place, between the devil and the deep blue sea, between two fires, on the edge of a precipice, on the brink of a precipice, on the verge of a precipice, on the edge of a volcano, in the lion's den, on slippery ground, under fire, not out of the wood, unwarned, unadmonished, unadvised, unprepared, off one's guard, tottering, unstable, unsteady, shaky, top-heavy, tumbledown, ramshackle, crumbling, waterlogged, helpless, guideless, in a bad way, reduced to the last extremity, at the last extremity, trembling in the balance, nodding to its fall, threatening, ominous, illomened, alarming, explosive, adventurous, incidit in Scyllam qui vult vitare Charybdim, nam tua res agitur paries dum proximus ardet.
N hopelessness, despair, desperation, despondency, depression, pessimism, pessimist, Job's comforter, bird of bad omen, bird of ill omen, abandonment, desolation, resignation, surrender, submission, hope deferred, dashed hopes, vain expectation, airy hopes &c, forlorn hope, gone case, dead duck, gone coon, goner, bad job, bad business, enfant perdu, gloomy horizon, black spots in the horizon, slough of Despond, cave of Despair, immedicabile vulnus, hopeless, desperate, despairing, gone, in despair, au desespoir, forlorn, desolate, inconsolable, broken hearted, unpromising, unpropitious, inauspicious, ill-omened, threatening, clouded over, out of the question, not to be thought of, impracticable, past hope, past cure, past mending, past recall, at one's last gasp, given up, given over, incurable, cureless, immedicable, remediless, beyond remedy, incorrigible, irreparable, irremediable, irrecoverable, irreversible, irretrievable, irreclaimable, irredeemable, irrevocable, ruined, undone, immitigable, lasciate ogni speranza voi ch'entrate, its days are numbered, the worst come to the worst, no change, no pause, no hope, yet I endure, O dark, dark, dark, amid the blaze of noon, mene mene tekel upharson.
N threat, menace, defiance, abuse, minacity, intimidation, denunciation, fulmination, commination, gathering clouds, threatening, menacing, minatory, minacious, comminatory, abusive, in terrorem, ominous, defiant, under the ban, Int, vae victis!, at your peril!, do your worst!.