The word 'thimbleful' is a compound noun formed within English. Its first element, 'thimble', originally meant a small protective cap worn on the thumb during sewing. By metaphorical extension, 'thimble' came to signify any small, cup-like container. The second element, '-ful', is a productive suffix denoting the amount that fills a specific container. Therefore, a 'thimbleful' literally means the amount that fills a thimble, emphasizing a minuscule, measured quantity. This term is most commonly applied to spirits or medicines, humorously or precisely indicating a very small dose.

Structure: thimble (small cap) + ful (full of) thimble (from Old English þȳmel, a derivative of þūma 'thumb', referring to a small cap for the thumb/finger); ful (from Old English -ful, an adjective-forming suffix meaning 'full of', later used to form nouns denoting a quantity that fills something).