* ftnl. - This is perhaps the most common abbreviation, but it's not widely used and might be unclear to some.
* bi-wkly. - This is a more straightforward option, as it explicitly means "twice a week," which is the same frequency as fortnightly.
* 2wkly. - This is a shorter and more modern abbreviation that is becoming increasingly common.
Ultimately, the best abbreviation to use depends on the context and your audience. If you're unsure, it's best to spell out "fortnightly" in full.