'unhappy phrase' is correct and usable in written English.
You can use it to describe a phrase that is sad or depressing, such as "We will miss you" in a goodbye speech. For example, "The crowd was silent as the speaker shared his unhappy phrase."
"Not unhappy" – the phrase brings to mind Duckworth's conversation with her father at 17, when she argued for happiness, he for accomplishment.
There are pedantic quirks, including a brief discussion over the correct use of "whence", which make unhappy turns of phrase stand out all the more: "The list of signatories was long and staunch".
"You sounded great," he said, but Gordon was unhappy with her phrasing and wanted to redo the vocal.
In his tenderest phrase, Milton (whose own first, unhappy marriage must have been instructive in these matters) writes, "In God's intention, a meet and happy conversation is the chiefest and noblest end of marriage".
France is going through a long-term political existential crisis, summed up by the morose phrase, heard often of late, "France is unhappy".
"If you only knew how unhappy I am," Konstantin upbraids Nina, repeating the same phrase to his mother when he has lost Nina's love.
Sir Richard Branson, the man who, more than any other, sums up the phrase "lower the retirement age now", is unhappy.
Mr Szajer says that this is covered by the phrase "any other circumstance whatsoever", but gays and liberals are unhappy.
To borrow a phrase that one of those writers would use elsewhere, the unhappy souls who inhabit "Beyond the Horizon" and "Spring Storm" are destined to live out a life sentence of "solitary confinement inside their own skins".
Romney repeated that promise this week, after deploying some extraordinary turns of phrase about the state of the Olympics, ones that made the British unhappy.
