Once formed, never changes; describes something fixed and unchanging.
成制定，形成。一经形成，不再改变。
《礼记·王制》一成而不可变，故君子尽心焉。”
She follows the same routine every day, never deviating from it.
Learn the next connected idiom: 一成不变 (yī chéng bù biàn) → 变本加厉 (biàn běn jiā lì)
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