Salonfähig is a compound adjective made up of ‘salon’ and ‘fähig’ (able, capable, competent), and translates roughly as ‘socially acceptable’.
It stems from the 18th Century trend for salons, which began in France as groups of liberal middle and upper class women met to discuss topics including literature and politics.
If something is salonfähig, it is ‘salon-able’. In other words, it is deemed suitable for the salon and therefore socially acceptable.
German women also began to attend salons in the 18th Century, with one of the most well known figures being Henriette Herz who set up literary salons with a group of emancipated Jews in Prussia. She established the tradition of salons amongst the Berlin bourgeoisie.
Heute sind Dinge salonfähig, die früher ein Skandal gewesen wären.
Today, things are socially acceptable that would have been a scandal in the past.
Das ist kein salonfähiger Witz.
That is not an appropriate or socially acceptable joke.