It's a term I first heard used by some trans* folk of my acquaintance, to whom "passing" means literally being able to pass as a male or female. Since then it's cropped up in disability circles in a similar way; passing for normal, hiding your non-normal ways and appearing to be a non-disabled, neurotypical member of society.
But I often can't, or don't, or a little of both.
I suppose in one way I'm a master of passing, since I sailed through life undiagnosed for a quarter of a century. But those were very hard times. I was often lonely, confused, hurting and frightened, and I'd reached the point where I couldn't pass any more. The pressure of passing, the exhaustion of trying so hard day in day out to be normal, had worn me down to the point that my life was coming apart at the seams, crumbling into the sort of ruin you'd expect of a building erected on a foundation of shifting, sliding, treacherous sand.
I'm very 'out' about having Aspergers. I bring it up in job interviews, when meeting new colleagues, when joining groups or turning up for an exercise class... Not in a special snowflake way, but because I think it's best to get the disclosure out of the way as soon as possible. That way, it's on the table before people make their own negative assumptions about my eye contact/clothing choices/stimming/bluntness.
The other reason I'm very open is that I refuse to force myself to act normal. I've tried, and that way madness lies in quite a literal sense. I refuse to torture myself by suppressing stimming, forcing eye contact, struggling through sensory hostile environments, and attempting things I cannot do due to the buffet of odd physical problems that come with the Aspie territory. These things wouldn't improve my lot in life any, the only benefit would be to the neurotypical people around me - by pretending to be normal, I save someone else the discomfort of having to deal with and make allowances for someone different. Someone Other.
But we are all someone's Other. There's someone out there who hates your skin colour, your religion, your genitals, your choice of sporting team and your decision about whether to eat meat. But that's their problem. You can't live your life in a way that makes other people happy, because there are too many people with too many agendas, and in doing so you're handing all your power, your choice, your self, over to them.
These days I'm keeping my power, my self-determination, my who-I-am.
And that's the opposite of passing.
In passing: a note on the use of "trans*"