adverb
respectfully; faithfully; with reverence
Formal, somewhat literary adverb used to describe acting with deep respect, sincerity, or devotion. Often appears in written or ceremonial contexts.
彼は師の教えを拳拳と守り続けた。
He faithfully continued to uphold his teacher's teachings.
社長は社訓を拳拳と唱えた。
The company president reverently recited the company motto.
謹んで is a more common formal adverb meaning 'respectfully' or 'humbly', used in speeches and letters. 拳拳 is rarer and carries a nuance of devoted, almost reverent adherence.
真面目に means 'seriously' or 'earnestly' in everyday contexts. 拳拳 is much more formal and implies a respectful, faithful attitude rather than just seriousness.
The exact derivation is uncertain. The kanji 拳 (fist) is used here for its sound rather than its meaning, likely as ateji. The reduplicated form 拳拳 (けんけん) may be related to the classical Chinese expression 拳拳服膺, meaning 'to hold firmly in one's heart with reverence'.