Last quarter moon

Shrinking: it is the waning Moon. After full moon, the Moon's right side goes dark first, so in the Northern sky the late-month Moon is lit on the left, like the letter C. That is ☾.

Its partner ☽ is lit on the right, like a D, and stands for the waxing Moon. Unicode calls them first and last quarter moon, even though both are drawn as thin crescents: strictly, last quarter is the half-lit Moon, and the C-shaped sliver is the waning crescent that follows it. In astrology the Moon is traditionally written ☽.

Why do people put ☾ in names and posts?

Mostly for looks: a small, elegant moon. As a plain text character it often fits where emoji don't, in usernames, game nicknames, bios and headings, and it normally takes the colour of the surrounding text. It sets a night, dreamy or mystical mood, often next to stars like ✩ or ☆, and some readers of tarot or witchcraft use it as a general Moon sign. It has no widely known rude or hidden meaning.