This is a domestic Brasilian airline. They have a lot of small
airplanes, doing more or less small hops with a lot of
turbulence. Hence there is a need for something definitely waterproof,
and also can take quite large volumes of uhm... content, as well as be
used more than once, in case of shortage of bags.
Thankfully it is not transparent, but when liquid is put in this bag, the appearance of it should be quite interesting and inspiring for co-passengers. The strings used to tie the bag shut, of course provides a possibility for opening it at a later stage and continue the process of emptying your stomach.
I have to thank Jan Sinstadt for this contribution.