For instance, an X-car diehard will stick at X-cars even it has proved that X-car has poor quality.
Please do not abuse this word---diehard. If you need have sex three times per week or every night, that means you are highly sexually active. But you are not a sex die-hard. Likewise, you may be a money maniac,
but you are not a money die-hard.
In any case, a hard is not prone to accept new things.