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.
by chaise longue April 09, 2024

Guy: So we could go get lunch somewhere, where do you want to eat?
Girl: Ooh I know! We could pack a lunch, roll the windows down and have a backseat picnic
Girl: Ooh I know! We could pack a lunch, roll the windows down and have a backseat picnic
by Harrietbubson September 10, 2020

Somebody who is not currently participating in a smoke session, but is actively trying to encourage the participants to get higher.
Person 1: "Take another hit of the bong right now."
Person 2: "Why are you telling me to smoke more? You're not even smoking."
Person 3: "Looks like Person 1 is being a backseat toker."
Person 2: "Why are you telling me to smoke more? You're not even smoking."
Person 3: "Looks like Person 1 is being a backseat toker."
by Riton July 21, 2022

Bart Simpson has been backseat coaching his kid to play the overload when the rest of the team is doing an umbrella.
by Johnny not-sville November 27, 2013

(MarioKart def)
Where someone shouts at you, giving you unwanted advice, when your just having a bit of fun.
Where someone shouts at you, giving you unwanted advice, when your just having a bit of fun.
by I_wonder who wrote_this June 26, 2020

When you poop in the back of an Uber, often as a sign of disapproval for your ride or driver, but in some rare cases it can be used to show affection.
Ryan: My Uber sucked.
Cody: Oh, did you give a bad rating?
Ryan: Nah, I just left a Backseat Klondike Bar.
Cody: Oh, did you give a bad rating?
Ryan: Nah, I just left a Backseat Klondike Bar.
by Lil' Pukey February 26, 2018

by Amanda the master December 23, 2017
