Effing is a rude word people throw in when they are angry, to make the feeling hit harder, without saying the stronger swear that starts with F. You will hear it in front of a thing (that effing door will not open) and in front of how something feels (I am so effing tired). It is not a clean word. It is still a swear, just half-covered: speakers say the letter F, or write “effing,” so the full word is not quite in the room. Some people feel a little safer doing that; some people feel it is the same punch with a thin cloth over it. You will meet it in films, in angry talk, and in writing that wants to show heat but stay just this side of the worst word. Though actually, it does not make the anger smaller. If someone says “effing,” they are usually as cross as if they had said the other word. In British talk you may also hear “effing and blinding,” meaning swearing a lot. This is not a classroom word, not a grandparent word, and not a job-interview word.