for several nights; night after night
Used to describe something continuing or happening over multiple nights, often with a sense of repetition or duration.
Sleepless nights continued for several nights.
He had the same dream night after night.
Combination of 幾 (iku, 'how many') + 夜 (yo, 'night') + も (mo, 'also/even'). The exact historical derivation is uncertain, but the structure is a common pattern for expressing 'many' or 'several' of a time unit.