丑三つ時

Etymology
Compound of.

In the old traditional Japanese method of timekeeping, the day was divided into twelve of two hours apiece, one for each of the twelve branches of the zodiac. One koku could be divided into four equal periods of thirty minutes apiece.

Noun

 * 1)  the time between and 2:00 AM and 2:30 AM, or between 3:00 AM and 3:30 AM, depending on the timekeeping tradition in use
 * 2)  the middle of the night, late night