[yaowenjiaozi]

While 'naive' translates to 天真, 天真 doesn't just mean naive. Baidu Baike [1] lists three meanings. Going by the cited references, <<庄子·渔父>> is the oldest and my guess is "真者，故受於天也，自然不可易也。故圣人法天贵真，不拘於俗" is the etymology of 天真.

[1] https://baike.baidu.com/item/%E5%A4%A9%E7%9C%9F/1419