"White Knuckling" refers to alcoholics/addicts who are using only willpower to stop drinking and are not using any other type of support system. Basically it means walking around with untreated alcoholism/addiction but not drinking/using. Often white knuckling it will eventually lead to relapse.
And no it is NOT an AA term. And yes there are times when I am not connected to the (fill in the blank) program I use to stay sober that I am "white knuckling it". I don't think it's meant to be a derogatory term, just a term to explain what it feels like to be sober but miserable because one is not doing anything to make their life better.