I have heard words like パイセン instead of 先輩. Is this commonly used? Is this applicable to all words or only certain words? Some other examples: しくよろ instead of よろしく
I take the liberty to comment on this even though I am not going to say anything related to Japanese...
In French, there is a similar way of talking that is called "Verlan", in which you invert syllables to confuse people (well originally, mostly to confuse and not be understood by the police). In spoken language, it dates back to the 50s and it is quite widely used nowadays, in everyday words, by people who are roughly aged below 40-50.
It is so very interesting to know that it also exists in Japan ! :) Thank you for sharing !