One of the foundations of trademark law is that certain words must remain in the public domain so that entities can properly describe their goods to the public. For example, I could not own the term "law firm," nor can anyone else.
Genericness can occur through "genericide" when a trademark becomes the commonly accepted word for a particular good. "Aspirin," "Thermos," "Escalator," and "Zipper" all met this fate, ironically due to their own commercial success.
Genericness may also exist at the outset of the branding process, which happened to Snyder's, the well-known pretzel commpany, and the manufacturer of a tasty treat they attempted to trademark as "Pretzel Crisps." Snack goliath Frito-Lay immediately took offense, arguing that "Pretzel Crisps" was a generic phrase that should be available for all snack manufacturers to use to describe their yummy goods.
Eleven years into this dispute, a North Carolina federal court has ruled that a "Pretzel Crisps" brand is a generic "common term" that one company cannot "monopolize" with trademark law. The case may not be over yet, with an appeal possible.
This eleven-years-and-counting ordeal is a poignant reminder that the strongest trademarks are those that are arbitrary, and that it's best to veer away from common terms when choosing a new brand name.