carry a torch
carry a torch or carry the torch {v. phr.}
1. To show great and unchanging loyalty to a cause or a person.
Although the others gave up fighting for their rights, John continued to carry the torch.
2. {informal}
To be in love, usually without success or return.
He is carrying a torch for Anna, even though she is in love with someone else.
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Source: A Dictionary of American Idioms