What is bming and machine gunning?

[THE POOPOOPEEPEE MAN]

Title
Also what is snowballing and sand baging?

[Mr. K]

BMing

Machine Gun:
STBFL + Unrelenting (killed by devs)
Huntress with two fast hatchet wind-up addons.

[Rim Carrey]

Bming is slang for "bad manners".

Snowballing is when the killer or a certain in other games just has such a good start or they pick up heavily and they turn things into their favor (which usually leads to them winning).

Sand bagging is farming your teammates from hooks or running the killer to them so the killer goes for them instead, basically..assisting in killing your teammates.

[Mringasa]

Snowballing - A Killer taking control of a match and it going downhill fast for the Survivors. Generally starts with 1 hook, then continues on until it's a 3/4k.

Sandbagging - A Survivor who blocks another Survivor's path (intentionally or not) and causes them to get hit or downed by the Killer. Also refers to Survivors who perform multiple fast vaults or cause other noise events near another Survivor to get the Killer to come and Kill them.

[Donut Steel]

BMing or Bad Manners is emoting or otherwise displaying body-language on your character in order to taunt the other side. This can also include offensive end-game chat.

Machine-gunning no longer exists: it was the tactic killers used by stacking two perks together which decreased the melee cooldown considerably in order to prevent hook-rushes from succeeding as much. The devs did nothing about hook-rushes, but over-nerfed the perks Unrelenting and Save The Best For Last beyond what survivors had even asked for.

Snow-balling is not directly related to player in-game behaviour, but a passive feature of game design where if one side is ahead then they are more likely to not only stay ahead but push into the lead even faster in a self-reinforcing cycle. In DbD this takes the form of a survivor dying very early on, drastically reducing the chances for the remaining three survivors, or multiple generators being finished before the killer has made any progress in sacrificing survivors.

Sand-bagging is specific to survivors; when the killer is persuing one who is then hindered by another survivor, making it more likely for the killer to catch their target. Sand-bagging is frowned upon heavily and some killers will even switch targets to punish the sand-bagger.

[FiniteThrills]

BM = Bad manners. Doing things to intentionally anger or humiliate the opponents.
Machine Gun = A build that let's you attack twice in very quick succession.
Snowballing = When one event causes further events to happen faster or in greater magnitude. The easiest to explain version is a killer camping a hook, and all 3 other survivors coming in to save and getting downed in turn. A 1k has now snowballed into a 4k. This isn't limited to camping, that's just the easiest visualization.
Sand-bagging = Actually has 2 meanings. DbD wise it generally refers to a survivor who intentionally gives away other survivors, or blocks them from an escape route, leads killer to them intentionally, unhooks them right in front of killer.
In other places (racing, sports, or pool) it generally means pretending to be much worse than you actually are, up until you turn the event around completely. Pool sharking is a version of this.