A “shy voter” is one who does not admit to supporting a certain candidate to pollsters, but still votes for that candidate in the election.
The term comes from the “Shy Tory Effect,” a phenomenon that found British conservatives greatly outperforming their poll numbers.
Shy voters seem to not make up a large percentage of the voting population, and have not been found to affect an election. However, the idea of the “shy Trump voter” was talked about in the 2016 election as a means to explain how Trump outperformed polls.
Harry Enten: “The ‘shy Trump’ theory relies on the notion of social desirability bias— the idea that people are reluctant to reveal unpopular opinions. So if the theory is right, we would have expected to see Trump outperform his polls the most in places where he is least popular — and where the stigma against admitting support for Trump would presumably be greatest… But actual election results indicate that the opposite happened: Trump outperformed his polls by the greatest margin in red states, where he was quite popular.”