When something is backdoor, it is secret, or you are not saying out loud that this is what you meant to do. A backdoor deal is one people make quietly, off to the side. A backdoor way into a job or a school is a way that is not the open, official one. The picture comes from a real back door: you slip in where fewer people are watching. Though actually the same word is also the physical door at the back of a building, and computer people use a backdoor for a hidden way into a system — same picture, different room. In this use it often sits in front of a noun: a backdoor agreement, backdoor methods. It can sound critical, as if someone is avoiding the fair route.