In IRC:
It means: "Private Message"

In time

it means: "post meridiem" (which is afternoon)

The abbreviation for the phrase “peg me.” It can be used in a variety of ways. To peg someone refers to an act of lovemaking in which a woman with a strap on penetrates her male partner anally. pm is to simply ask if your partner can peg you.

Private message me dickhead

The abbreviation for a Phantom Watermelon

For men: a thirsty call for noods. Like a baboon makes its butt more red to attract a mate. The Kik male uses “PM open” in their username and usually a torso to try and lure women in for the noodles.
For women: a risky move on Kik. 83% of women with “PM open” are actually males (see above) which make the other 17% just wanting some deep.

Da stations-band dat you listen to after 12 noon.