Originated on AR15.com when a member (Shawnofthedead) was invited to a threesome that included a male and female and started a thread. He, in haste and against the advice of many members, said "fuck it im foing" and was never heard from since.
An obvious typo has now transformed the word "going" to "foing" on the ARFCOM boards.
When you're Fucking Going, you are FOING! Comes in specially handy when there are children around and you want to emphasize the fact that you are going but can't use the word fuck.