shadowy past; murky past; dark history
Refers to a person's or group's past that is hidden, shameful, or unpleasant. Often used when discussing someone's background or history that they would rather not reveal.
See also: 暗い
He seems to have a shadowy past.
The politician's dark history was exposed.
Compound of 暗い (kurai, 'dark, gloomy') and 過去 (kako, 'past'). The figurative use of 暗い to mean 'obscure' or 'shameful' is a natural extension of its literal meaning.