He describes natives of the north of the Yucatan peninsula (probably close to or Mayans) as wielding "cotton weapons" (armas de algodón). I'm guessing it's not maces with literal cotton heads?
Edit: one person responded (but whose comment is just gone now) and said it might refer to armour, which makes sense.
Spanish speaker here, it refers to quilted cotton armor. Almost all the natives used such armor (as they predominantly used blunt weapons over cutting ones) and in many parts of the book Diaz writes that they were given as gifts to the Spaniards. They were more effective than they might first seem (very tightly packed) and many Conquistadores adopted their use.