What is the meaning behind 华胄 ? I found the sentence 他是华胄，所以气质不凡. It is been translated as "He is the offspring of a noble family, thus his character is not a common one". I also found for 华胄  the translation "offspring of the huaxia" and simply "chinese".

Verbatim, 华胄 would be "chinese helmet", "flowery helmet", "illustrous helmet".

Obviously the word is a metaphor,
what was is its original meaning and what does it imply culturally as a metaphor ?
胄 (由 + 月) means descendants, whereas 冑 (a different character, 由 + 冃) means helmet. Since they look similar, they got merged.

华胄 means either of Han lineage or of noble descent.

There is no word 华冑, which would mean Chinese helmet.