Missinterhearing is when someone says something and you don't quite hear what they have said, but instead of asking them to repeat it you respond with what you think you have heard. Old people are known to do this frequently.
Housemate A: It's three days till we get internet!
Housemate B: WHAT? Did you just say that there are three days till we get INTIMATE? What the hell!!!
Housemate A: Dude, you totally missinterheard me.
Person A: Tonight I'm going to wear my sluttishly high heels...
Person B: What on earth are Cottage Pie Heels??