I would hope that the Patent & Trademark Office wouldn’t allow the generic term “serif” to be trademarked as the name for a font. I’m pretty sure it doesn’t pass the litmus tests.

There already is a family called TheSerif. I have no idea if it is trademarked, or could be (in the US). On that basis, I'd bet you could use one word for the name, say BigSerif.

For the sake of argument, let us say that the patent office lost it's mind and allow a generic common use word to be registered. Stranger things have happened. If not serif, some other generic word, like Heavy or Big, maybe Square.

Also, what is the official litnus test?

I don’t know if there’s an “official” litmus test. Trademarks are characterized as “strong” or “weak” based on a number of criteria. Generic marks are considered quite weak and must have other aspects going for them to qualify for protection.

Some basic info here: http://www.nolo.com/legal-encyclopedia/qualifying-trademark-protection-f...

But really, you’re asking questions that should be asked of an IP attorney.

Who has money to talk to an attorney. It is a hypothetical question, opinions, thoughts and so forth, just for the sake of argument. No legal advice necessary.

If you are contemplating a dubious name for a typeface, then you should invest in legal advice.
You don't have to be a lawyer to know what would be problematic.
Many foundries have the money.

That's just the thing, some names don't seem dubious. The word Poster for example. I guess it falls into the category of a dumb question. I'm good at those.

You could register the trademark "Xxxx Serif" but your registration would have to make clear that you are disclaiming any rights to Serif on its own. I own the registered trademark "VF Sans" but certainly have no right to the word "Sans" on its own. However if something is too descriptive of what it is, you will not be able to trademark the name. I once attempted to trademark the name 5 X 7 for a typefont made up of dots on a 5 x 7 grid, and that application was rejected.

That said, hire an attorney!

The only certainty I’ve found within the U.S. legal system is that there are no certainties. This was made abundantly clear to me when I sat on a jury in a civil case. The law is very objective. The interpretation of it is not. That said, you could hire an attorney, but I don’t think that’s necessary at this point in your hypothetical legal pursuit. I have a client who obtained the coveted “circle-R” from the USPTO for a new product. He also successfully pursued a cease and desist from someone who was diluting one of his existing trademarks by using a name that was too similar, a name which at that time didn’t have the official ® protection. He used an online legal service along the lines of legalzoom.com to acquire the official USPTO registration at a cost of around $600 to $700. Hiring your own lawyer to pursue getting the USPTO trademark registration usually cost around twice that or more.

Without going the official route of registration and getting the ®, at some cost, you can declare a trademark by just placing the TM indicia on your product. I’ve gotten into mild dust ups with some brand managers over that because it’s not even legally required any more to use the TM, ©, or ® indicia. They insist though that the lawyers will have an easier time pursuing a case if the outright claim to ownership is staked on the logo or product. You can just stick the TM on anything as a declaration that you claim the trademark. Without the official registration though it’s easily challenged by someone with a registered trademark who believes you’re infringing on theirs.

A case in point similar to your hypothetical example. The air cargo company here uses two generic terms, Federal Express as a registered trademark. I tend to think others in the air cargo company could use other generic combinations of those generic terms and withstand any legal challenge, e.g, Big Express, Blue Express, Federal Air Freight, etc. However, others in different businesses have been hammered by the FedEx lawyers for infringing on that registered trademark. One of the cases that got nationwide attention along those lines was the coffee shop in Syracuse that decided to trademark the name Federal Espresso. They were overwhelmed by FedEx’s lawyers pursing legal action and I think are now called Freedom of Espresso.

Take the above for what it’s worth. I’m no lawyer, but do have to play in the same sandbox with them from time to time. The question in fighting big trademark issues usually doesn’t come down to right or wrong, rather who has the bigger shovel in the sandbox. The question for you (hypothetically) is how big of a shovel do you want to buy to own that name.

It’s also worth pointing out that with trademark law, it only last as long as you legally pursue ownership and work actively to protect against infringement, i.e., they do go stale and expire with neglect. That’s why big brands like Coca-Cola, FedEx, Apple, etc. aggressively go after anything and everything that remotely looks like an infringement.