The English word “thin” is also a tricky word when you want to translate it into Japanese.
When somebody is “thin” as in “skinny”, we say 痩（や）せた or 痩（や）せている as well as 細（ほそ）い.
When a thread or something long is thin, we say 細（ほそ）い.
When a book or something flat is “thin,” we say 薄（うす）い. 薄い can be used for colours being “weak” as well.
When an alcoholic drink is “thin,” we say 弱（よわ）い, which is not difficult to remember as we can the word “weak” in English, too.