noun
sitting while waiting; eighteen-day-old moon
The first meaning is literal: sitting and waiting. The second is a traditional lunar calendar term for the moon on the 18th day of the lunar month, named because one sits waiting for the moon to rise later in the evening.
居待ちの月は、夜遅くに昇る。
The eighteen-day-old moon rises late at night.
昔の人は、月の出を居待ちで待った。
People in the old days sat waiting for the moonrise.