Behaving in a childish manner; acting like a child or showing childlike qualities.
From 'child' + '-ing' (a present participle or gerund suffix). This archaic form means 'acting as a child would,' with the '-ing' creating an active, present-tense quality of childishness.
Words like 'childing' show how English speakers once could make adjectives more fluidly—we'd add '-ing' to nouns to mean 'being like that thing,' but modern English has mostly restricted '-ing' adjectives to verbs, making 'childing' seem quaintly archaic.
Complete word intelligence in one call. Free tier — 50 lookups/day.