Manspreading is the practice of sitting with legs spread wide apart on the subway or bus, making the seats on either side unavailable to other riders. It is annoying and rude, and, as with mansplaining, is (by definition) a practice limited to men.
Some public-transport riders have taken to snapping photos and posting them online in an attempt to shame offenders. And, according to Wikipedia, New York's MTA and Seattle's Sound Transit have started poster campaigns to encourage men to use one seat at a time.
As always, pointing out the problem has led to a backlash in which super-sensitive men respond that women should stop putting their bags or other belongings on the seat next to them, thereby depriving other passengers of space.
I've got to say, though, that it's not just the taking up of space that is offensive in manspreading; the outspread display is just plain disgusting (for me and many other women, and men).