Where to begin...
How about one time when my aunt & uncle were visiting when I was still a kid. He's a science teacher and was excitedly talking about magnets over the lunch table before musing over something he didn't understand about our planet's electromagnetic field. Having studied astronomy extensively by that point, like the little nerd I was, I offered him the answer, stated simply while smiling, believing I was being helpful. He informed me very sternly I was wrong (with no explanation) before letting out a little bristled laugh, no doubt put off by the 10 year old talking physics.
I said nothing but slipped away, letting the adults continue their conversation while I consulted my books. I returned with my oversized London Planetarium book, which seemed roughly the same size as me at the time, having no choice but to drop the hefty open book I'd been carrying against my body on the table with a thud. I then stood holding my finger to the answer before him. I still genuinely thought I was being helpful, information is a gift...
The chatter in the room died off, the vibe suddenly cooled, even before my uncle gave it a skim read. And afterwards his facial expression showed him to be very unimpressed. He acknowledged nothing but his silence spoke plainly. My INFP mother had to fuss over dishes and quickly take them away so she didn't get caught smirking. The other adults were being very stiff & awkward in the uncomfortable silence. I suddenly realized I was perceived as having done something wrong. Not that I could care, who doesn't value truth above all... it seems me skipping away, leaving my big book and ignoring their social cues proved to them that I was a meanie. My uncle never came back to visit, ever.
I guess that was my introduction to Fe. It took me a while to learn that speaking truths, which seems so natural to me, is frequently far from normal or charming or even safe. And yet, I still can't speak without being bluntly honest. Or at least I hate not being that way. And now that I'm older, with Te well developed, it isn't just which words I choose that invokes this meanie-me phenomenon, it's how I enounce those words too.
As I've described before, I once got stuck doing lab work with a couple of feeler girls. Thinking I'm helpfully moving things along/setting the pace, in addition to my usual serious/confident/efficient Te speech I added an energetic, let's-get-this-done aspect to the tone too when I said; 'you do X, you do Y, I'll do Z and then we all do B & C together'. I then paused waiting to hear what they think of my suggestion and to find out if they have a better one (to me, a 'how about...' preface was clearly implied. These negotiations at the beginning of these labs is normal).
Upon hearing my words, one girl falls back into her chair and slumps over as if she's been physically hit. The second one rushes to tend to the wounded. While the first one starts faux crying the second angrily tells me; "You can't tell us what to do!" [Thinking: well yes, you clearly prefer to be manipulated into action...] So now I'm annoyed, so I give them INTJ biting tongue/Severus Snape style (from Harry Potter): 'That. Was. MY i-dea. Do, youuu. have-one, of. Your. Own?'
They emotionally hijacked our lab time for another twenty minutes before going with my original plan. There was no shortage of meanie looks shot my way that day. After that experience I try to stay mindful of my tone, but I can't help but speak in a fairly forceful (confident/direct/efficient/not soft) way when I'm in Te 'work mode'. I also have a habit of cutting off over-talkers in a work setting, telling them to "give me actionable information". No doubt that's considered mean but not more than their time wasting imo.
The other default meanie factor is my 'INTJ face'. I thought it was just 'serious/focused' but my ISFP friend informed me just the other day that I have "resting bitch face" when out in public but the moment the door closes and I'm in private with only people I'm close with, my face becomes "child-like" -- she means open/relaxed/happy. That certainly mirrors how I feel.
So, to summarize, I'm mean in the way I'm bluntly honest/value truth above feelings. I sound mean. I even look mean apparently, lol. I don't think this list would be complete without mentioning how mean my dominant Ni is to my ISTJ father's dominant Si. Just thinking of future possibilities is rude, but to have the audacity to try new things, to default to 'yes!' when his is 'no!' -- no wonder I get death glares in the hallway. I guess a 'meanie' like me has it coming... Good thing I don't care -- but then, perhaps that's my 'meanest' quality of all; indifference. "She doesn't care!"
I think some consider us independent types 'mean' (selfish) by default without us having done anything.