Literally: heavens – true – inflamed – involuntary
Alternately: When a person’s thoughts and feelings show on the surface of their words and behavior.Simplicity; naivete; unaffectedness. Wearing one’s heart on one’s sleeve. Keep in mind that in contemporary Japan, this compound and the quality it describes are heavily associated with (stereotypes of) femininity and youth.
Notes: This is a compound of compounds. 天真 refers to a person’s character that is natural, unguarded, or “pure”, while 爛漫 is something like “shining forth.”
In some cases 爛 may be replaced with homophone 瀾 (“large waves,”), which we’ve actually already seen once. That said, the former (with the fire radical 火 rather than water radical 氵) seems to be preferable. Also, replacing 真 with homophone 心 (“heart”), or 漫 with homophone 慢 (“ridicule,” “laziness”) is an error.
This phrase comes to us from the Chuo geng lu (『輟耕録』, Japanese Tekkouroku), an extensive collection of all sorts of writings from mid-14th century CE Chinese scholar Tao Zongyi (陶宗儀, Tou Sougi).