home truth: A key or basic truth, especially one that is discomforting to acknowledge. For example, It's time you told a few home truths here, such as where your campaign finances actually came from. This expression uses home in the sense of “the very heart of a matter.” [c. 1700]

I remember coming across the phrase as a teenager while reading Georgette Heyer's fabulous Regency romances. It's a Brit thing.

Just in case you were wondering.

Anhata
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