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Having no ground or foundation; unsubstantiated: groundless optimism. See Synonyms at baseless.

ground′less·ly adv.
ground′less·ness n.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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Without basis or foundation in fact:
The American Heritage® Roget's Thesaurus. Copyright © 2013, 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
References in classic literature ?
Your majesty's mind is disturbed by anger; otherwise you would be far from offending, groundlessly, the very one of your servants who has rendered you the most important service of all."
Geoffrey Delamayn, whose fair fame had been assailed (it was needless, the correspondent added in parenthesis, to say how groundlessly), was understood to have expressed, not only the indignation natural under the circumstances but also his extreme regret at not finding himself in a position to aid Captain Newenden's efforts to bring the anonymous slanderer to justice.
Of her father's being groundlessly suspected, she felt sure.
"The State Committee for Religious Affairs groundlessly refusing to register 'tenirchilik' upon the pretext that this religion may cause controversial issues among population," he said.
'If the courts continue to groundlessly and unlawfully issue summonses for the former members of the CNRP, and even the continuation of restrictions on the freedom to peacefully assemble, relations with the EU and US will not be good and measures to suspend the preferential trade systems will take place."
"Is my solidarity with Jews not based on unconditional compassion, empathy to those who have groundlessly suffered most and longest?
"They are suspected groundlessly," he told reporters on a conference call.
The court determined that if they were to deny the plaintiff's claim they would be "groundlessly excepting one type of medical malpractice from negligent liability." Id.
If they are really concerned, then they must probably influence their ally - the United States - to persuade the Americans to come to the negotiating table and start discussing the issue in a concrete way, and not accuse us groundlessly across the Atlantic," Lavrov stated, adding, "We are prepared for such a discussion."
Pakistan still does not recognise Armenia as an independent state because of its illegal occupation of Azerbaijani territories and supports Turkey whole-heartedly against deceptive and groundlessly false notions claimed by Armenia.