Die hard or die heart?

I just saw someone write that they were a "die heart" fan. I always thought the term was supposed to be "die hard" but I decided to google it just in case I was wrong.

Google was unable to give me a definitive answer as there were folk on both sides of the fence. Anyone know the true answer?

I think "diehard," "die hard," and "dieheart" are all different.

diehard |ˈdīˌhärd| noun [ often as modifier ] a person who strongly opposes change or who continues to support something in spite of opposition: diehard traditionalists | she was a diehard Yankees fan. ORIGIN mid 19th cent.: from die hard (see die1) .

"Die hard" is a phrase, not a word.

PHRASES die hard – disappear or change very slowly: old habits die hard.

"Die heart" is nonexistent in a legitimate dictionary, so all we can do is consult definitions that may have been randomly fabricated.

As Kris defined from UrbanDictionary:

One who is ridiculously overwhelmingly passionate about a specific thing, person, place, verb, or adverb. One willing to die for said cause, and as a result their heart would stop beating. She was a dieheart about the consumption of breakfast cereals. Thus meaning that she valued the consumption of breakfast cereals with great integrity.

die heart, also dieheart
Informal, after the expression die hard, esp., in contrast, not as a synonym/ variant.

See UD dieheart:

One who is ridiculously overwhelmingly passionate about a specific thing, person, place, verb, or adverb. One willing to die for said cause, and as a result their heart would stop beating.
She was a dieheart about the consumption of breakfast cereals. Thus meaning that she valued the consumption of breakfast cereals with great integrity.

cf. UD 2. die hard:

It means when you are determined to do something, similarly like Bruce Willis in the film Die Hard.
Can be used as a verb or a noun.