1.
early evening, early part of the night
The period shortly after sunset when the night has just begun, roughly 6 to 9 PM.
まだ宵の口だ。
It's still early evening.
The early part of the evening, roughly 6 to 9 PM. Suggests the night has just begun and there is plenty of time ahead.
Usage
Often used in social contexts — especially drinking — to imply "the night is still young." The phrase まだ宵の口だ means "it's still early in the evening" (so let's keep going).
Etymology
宵 (evening, literary) + の + 口 (opening, beginning). Literally: "the mouth/opening of the evening."