Recorded in the Ming-dynasty novel 《水浒传》 (Water Margin), Chapter 2: '朱武、杨春两个正在寨里猜疑,捉摸不定' — Zhu Wu and Yang Chun were in the stronghold full of doubts, unable to figure things out.
usage
Often describes weather, moods, or markets — anything fluctuating. Synonym: 变幻莫测 (biànhuànmòcè). Antonym: 一成不变.