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1. An unidentified flying object (popularly known as a UFO) in the shape of a dark triangle, typically with points of light in each corner. I know you think I'm crazy, but every spring, I see those black triangles appearing over my corn fields at night!
2. In Nazi Germany, a badge assigned to concentration camp inmates considered "asocial" or mentally unfit for work. Her clinical depression, which was vilified by the Nazis, meant she had to wear a black triangle in the concentration camp.
A sexual encounter or relationship between three people. Although exciting at first, the eternal triangle caused their relationship to suffer and ultimately end.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
(the) eternal triangle
a sexual or emotional relationship involving two women and one man or two men and one woman. (*Typically, a couple [man and woman] and another man or woman.) Henry can't choose between his wife and his mistress. It's the eternal triangle. I'm surprised Jane doesn't get tired of the eternal triangle. She goes out with Peter at the weekend and Jim during the week.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
A relationship involving three lovers, such as two women involved with one man or two men with one woman. For example, The plot of the murder mystery revolved around the eternal triangle of a husband, wife, and another woman . [c. 1900]
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
eternal trianglea relationship between three people, typically a couple and the lover of one of them, involving sexual rivalry.
Farlex Partner Idioms Dictionary © Farlex 2017
eternal triangle, the
A threesome of lovers, either two men involved with or vying for one woman or two women and one man. “Eternal” here simply means that this situation has occurred over and over through the ages. The term has been traced to a book review appearing in the London Daily Chronicle in 1907, describing a novel that “deals with the eternal triangle, which, in this case, consists of two men and one woman.”
See also: eternal
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer