I have only understood it to mean every two months. It's very confusing if it means both.

Or, what -t said!

Why does it mean both?

As I understand it, it's for the same reason we have flammable and inflammable. Its original meaning was "every two months," but people kept using it as meaning twice a month, and the powers that be gave up.

Signed,

My twice-weekly college newspaper was semiweekly, not biweekly

Someone explained to me the difference between flammable and inflammable once, and it made perfect sense, but then I forgot it.

It's possible that was a dream.

ita ! I've been doing the stairs at work, too. I started out only going down. Then I slowly started adding going up a floor at a time (I park on the third level and work on the seventh floor.) I've been going down every evening for a while and then I slowly started going up a floor at a time, week by week. I can make it 3-1/2 to the full four floors now every morning. I started doing that about a month and a half ago. Today, I only made it up two floors in the morning (the humidity was something fierce and the stairwells aren't air conditioned), but I went out for lunch with seven floors down and four floors up, so I feel I made up for my early morning wimpiness. I think it's a great way to build cardio endurance.

Someone explained to me the difference between flammable and inflammable once, a

They both mean "this shit will burn." The original word was inflammable, from the Latin for "to set fire to." People kept thinking the "in" meant "not," so people labeling dangerous things went with "flammable."

Semi means half. A semicircle is half of a circle. A semiannual publication comes out twice a year. Semimonthly means twice a month.

Bi- is always unclear and is best avoided because it just means "two", without indicating if you're halving or doubling.

This is why I don't buy the argument that we should accept definitions "everyone" uses, even when they're, wrong, wrong, wrong.

Why is using "two" to mean "half" okay?

It might have been inflammable and combustible. That would make more sense.

To be fair to the rise of "flammable", I have seen "inhabitable" used to mean "not habitable" in official documents and that is just confusing.

I have to go up the whole ten floors at once, because we have split elevators, and the eleventh floor is the first floor on the top half. So you can only get to it from the ground floor and the floors above it by elevator. No gradual working up to that!