To mislay something is to put it down somewhere and then be unable to remember where. The thing is not gone from the world — it is almost certainly in your house, in your bag, on some shelf — it is just that you and it are temporarily out of contact. That hope is built into the word. If you say you have mislaid your passport, you are saying you expect it to turn up. It is used about small, portable things you handle every day: keys, glasses, a phone, a form, a receipt, someone's letter. You would not mislay a car or a person. The form to watch is the past: mislay, mislaid, mislaid — like lay, laid, laid. Never "mislayed". Now, the honest part. This word is more British than American, and even in Britain it has an old-fashioned, careful feeling to it. Younger speakers mostly just say lost, or "I can't find my keys". You will meet mislay in books, in polite notices, and in the speech of older or more formal people — a hospital saying "if you have mislaid your appointment card", say, where lost would sound like an accusation. Knowing the word is genuinely useful for reading; using it in casual conversation with your friends would sound a bit bookish, and that is fine to know in advance.

一時忘了放哪、（東西）隨手放而找不到；口語常説「不見了」「忘記放哪」（不是真的遺失 = 遺失/弄丟）