A thimbleful is a very small amount of a liquid — as much as would fit in a thimble, which is to say almost nothing, a mouthful at most. It works like 'spoonful' or 'handful': the container plus -ful gives you the amount. In real use it is not a measurement anybody follows; it is a way of saying 'a tiny drop, and I want you to feel how tiny'. It comes up most often with strong drink — a thimbleful of brandy, of rum — either because that is all a polite person would take, or because someone is complaining that they were served too little. People also stretch it away from liquid, half-jokingly, to mean a tiny amount of anything abstract: a thimbleful of sympathy, a thimbleful of common sense. That stretched use is usually sarcastic. The word sounds slightly old-fashioned and slightly literary; a young person today would more likely just say 'a tiny bit' or 'a drop'.

一丁點（液體）、少得可憐的量。中文常說「一點點」「一小口」；帶點誇張、嫌少的語氣。