Other forms: nightmarishly
Something nightmarish is truly horrifying, something that would show up in a bad dream. Your nightmarish day might start with a car accident on the way to school and end with a fight with your best friend.
A chilling scream in the middle of the night is nightmarish, and traumatic experiences like an episode of bullying or nearly stumbling off the top of a cliff would also be nightmarish. You can also use this adjective to emphasize the unpleasantness of an experience in a slightly exaggerated way: "Ugh, that math test was absolutely nightmarish." Nightmarish things resemble nightmares, or terrifying dreams.