I have come across the word "cheersing", with an "s", as opposed to what I believe to be the correct form: cheering.
I think it comes from a misguided verbification of the exclamation "cheers!", as in the plural of the noun "cheer".
However since there is already a corresponding verb, "to cheer", I don't see why anyone would need to force a plural noun into a progressive verbal form like "cheersing".
Besides, it's impossible to conjugate and hilarious to even try:
Please tell me that cheersing is only a made up word.