When a player in any game chooses (on purpose) to not play their best. Normally this is because they are too superior, they want to hustle you, or they are too lazy to play their best with nothing on the line.

in corporate sales: when reps report a much lower deal opportunity than actually exists. reps do this so that if the deal doesn't go through they don't look as bad and so at the end of the quarter when the company has to make their number the rep isn't selected to perform a high pressure last minute sales job.

In gaming, the act of slowly leveling while otherwise increasing stats and currency in order to be stronger than other players at your same level; who have not sandbagged. Particularly useful in pvp focused games that pair players based on level.

The act of "sandbagging" includes, preventing a person(s) from doing a certian activity. Sandbagging also includes hastling a person.

The female version of tea-bagging. Dropping/slapping large breast in your partners mouth

to screw someone over; most notably, cock blocking someone
to sandbag someone

A military term used when an entire unit is given "extra training" (or in other words is punished) because of the actions of one or a few in that unit. This usually consists of filling sandbags and/or moving them from one location to another for no apparent reason, then moving them back.