Somebody is trying to do something bad to you or doing something you don't want to be don - get you fired, make you look bad, try to stop a project you want.. They are using facts and/or rumors to it. If you do something that looks bad - be late for work, forget some part of the project, make a big mistake - it will give your opponents or adversaries more arguments to use against you or your project. Figuratively, you would be giving them 'ammunition'. The image is one of warfare between you and them and the more ammunition your enemy has, the worse it is for you.