This word has been around for a little while but is picking up in usage. It is a portmanteau word blending man and explain and describes the way that a man will condescendingly explain something to a woman in the belief that she won’t or doesn’t understand and needs informing.
A recent example occured in the multi-award winning film La La Land in which the hero decides to give his new girlfriend a potted history of jazz music, which she of course couldn’t possibly have known about. Some commentators have denied this on internet saying that Ryan Gosling’s character was simply talking about his passion. You decide!