Explaining something in a slightly
condescending,
patronizing manner, while a) assuming the person you're talking to isn't
knowledgeable about the subject, b) likely believing you know more about it than you actually do, and c) thinking they must need to hear your wisdom.
Can be used as a suffix to indicate the topic being 'splained, e.g., dog-splaining, car-splaining
#1)
"I just got
a puppy and you would not believe how many people dog-splain to me how I should train it or what its breed is like. They all think they're expert animal
trainers."
#2)
Person A: "Hey man, I got a chance to look at your YouTube videos. You know, channels get more views when they have attention grabbing thumbnails. You might want to try that. Also, one way to get more engagement is to have an introduction that..."
Person B: "Dude, stop YouTube-splaining me. I run my own channel. You think
I don't know about all that stuff already?"