The Magic Bikini is a recurring outfit in the Dragon Quest series, worn by the female party members.
Dragon Quest III
It is a piece of armor that can be worn by any female party member. It has a defense bonus of +65 and provides a lot of protection against magic.
Dragon Quest VIII
It is an outfit for Jessica, which changes her appearance when worn. It can be purchased in Arcadia and provides a defense bonus of +50. It also reduces damage from Frizz, Sizz, and Bang-type spells by 15.