跟嬰幼兒有關（專業用語）
這一支就是畫面本身，也是這個字最早的用法：單純標示「屬於嬰幼兒這個階段」，不帶任何評價。
adjective · C1
Relating to babies and very young children. It is used in medical and scientific writing, and it goes in front of the noun.
嬰幼兒的
核心畫面
Picture a baby who cannot talk yet — the stage the word was built on. Medical writing still uses it exactly that way, as a label for that period of life. The other route takes the same picture and lays it over a grown person: an adult doing what only that stage would explain. Nothing about the picture changes; what changes is that it is now in the wrong place, and that gap is the insult.
可以想成一個還不會說話的嬰兒。infantile 最早就只是「跟這個階段有關」，醫學文字到今天還是這樣用。另一條路是…更多收起
可以想成一個還不會說話的嬰兒。infantile 最早就只是「跟這個階段有關」，醫學文字到今天還是這樣用。另一條路是把同一個畫面疊到一個成年人身上：一個大人，做出只有那個階段才解釋得通的事。畫面本身沒有變，變的是它出現在不該出現的地方；那個落差，就是這個字的殺傷力。From the word’s history
From Latin infans, a child who does not speak yet, plus -ile, meaning belonging to. So the word first meant simply of infants. The dictionary record dates the pertaining to infants use to the mid-15th century and the critical infant-like use to 1772, which is why the technical meaning is the older, more literal one.
來自拉丁文 infans（還不會說話的小孩）加上字尾 -ile（跟…有關的），所以它最早就只是「嬰幼兒的」。詞典紀錄把「跟嬰幼兒有關」放在十五世紀中，把「像小孩一樣」這個帶批評的用法放在 1772 年。也就是說，專業的那個意思才是比較老、比較字面的一邊。
Wiktionary etymology supplied in this evidence packet (source_ref kaikki:3f6f69a190036ae4:1139163449): Mid-15th century, pertaining to infants, from Latin infantilis, equivalent to infant + -ile, from infans. Sense of infant-like is from 1772.
Two branches
幼稚（一句批評）
把嬰兒的畫面蓋到成年人身上。字義沒變，位置錯了；錯位本身就是那句批評。
adjective · C1
Behaving, talking, or joking in a way you would expect from a very small child, said about someone old enough to behave differently. It is a criticism.
幼稚的