Backseat wanker

In essence, calling someone a "backseat wanker" suggests that they are not actively participating or contributing, instead, they are being unhelpful or critical from a position of relative ease or distance, akin to someone sitting idly in the backseat of a car while others do the work of driving. It's a derogatory term used to criticize someone's behaviour or attitude.

Let's say a group of friends is working on a project together, and they're all brainstorming ideas and putting in effort to get the work done. One person, let's call him John, sits back and offers no suggestions, doesn't contribute any work, and just criticizes the ideas and efforts of others without offering any constructive input. John is essentially being a "backseat wanker" in this scenario, as he's not actively participating in the group's efforts but is quick to criticize from the sidelines without offering any meaningful contribution himself.