fervent

From Middle English fervent, borrowed via Old French fervent from Latin ferventem, a form of fervēns, the present participle of ferveō, meaning to burn or be hot. That Latin root traces back to the Proto-Indo-European base *bʰrewh₁-, meaning to boil or brew.

The Latin verb ferveō could mean both literal heat and figurative agitation or zeal, and English fervent kept that figurative sense, describing passion or intensity of feeling rather than actual temperature.

It shares its Proto-Indo-European root, *bʰrewh₁- (to boil or brew), with the very idea of brewing, since both trace back to the same ancient sense of heating and bubbling.