My girlfriend and I were unaware of the term "mansplaining" but after reading this thread it is exactly what she means when she tells me "you're doing that condescending thing you do again".
We used to fight over this, because she felt belittled as though I assume she knows nothing about anything, and I felt defensive because from my point of view I thought I was trying to be helpful, useful, and knowledgeable, and I couldn't seem to understand in what way I was being condescending. So, with the godlike patience she possesses towards me, she continued to point it out each time I started doing it to her or our friends (I was not selective about target, male, female, I mansplain to EVERYONE) and slowly I have come to accept that I am, in fact, a condescending jackass way more than I ever realized. Now is where I get defensive and point out that quite a lot of the conversations I have with people are in the context of explaining why their computer or electronic device doesn't work and how to fix it, but the fact of the matter is that I simply fail to give thought to whether or not the person I am speaking to might already know what I'm explaining, and I then proceed to explain as though they had no idea.
We've been working on it for awhile now but I still do this constantly. I think part of the problem is I still get defensive sometimes when my girlfriend points it out, as though my knowledge and usefulness were being rejected even though I know that's not actually the case. I think this has caused her to only point it out when it's especially irritating to her, which means a lot of my condescending jackassery goes unchecked.
It is REALLY hard for someone like me to realize when I am mansplaining. I constantly battle a large, deeply ingrained case of SGiTR in myself, and I think it contributes to my ignorance towards my mansplaining. It's actually very rare I catch myself at it rather than my gf pointing it out to me. I've learned to simply shut up when she plays League of Legends, for example, because I cannot seem to resist giving her tips on playing or warning her of what the character against her is capable of, despite the fact that she is much better than me at that game.
Which also makes it more difficult in that context... I've been playing League of Legends for years, and played heavily the Warcraft 3 mod it came from (DotA), yet in the three months she's been playing LoL, my gf is far better than me at it. I can handle that, but it does make me more apt to try to mansplain to her in that context, as I taught her how to play and am both proud of how good she's gotten and humbled at how much better than me she already is.
Well, pain is learning. Hopefully she will keep beating me with the stick and I will grow and learn. This problem of mine I don't think I can tackle without someone verbally whacking me when I start doing it.
Anyway just my take on the OP. Thanks for this thread, I will show my gf when she gets home, hopefully she will enjoy having a fresh term to address it directly, rather than "you went into condescending jackass mode again".