The word "flight shame" is the translation of the Swedish term "flygskam". The latter became known in 2018 as the hashtag #flygskam via social media such as Twitter, Facebook and the like from Sweden and quickly gained a very large reach. Meanwhile, journalists pick up the word flygskam again and again when reporting on climate change in connection with air travel.
It is indeed the case that air travel is the most climate-damaging mode of transport. For example, an aircraft emits around 200 grams of CO₂ equivalents per person for every kilometre travelled. By comparison, a coach or train emits less than 50 grams of CO₂ equivalents. A flight therefore emits more than four times as much CO₂ per kilometre than a bus or train. For many Swedes, the motto for this reason is #stannerpamarken, which translates as "stay on the ground".