IPA: /ˈθɪm.bəl.fəl/
KK: /ˈθɪmbəlfəl/
A very small amount of something, typically the quantity that can fit in a thimble.
She poured a thimbleful of water into the plant pot.
Thimbleful is formed from "thimble" (from Old English "þymel", meaning a small cup or cover for the finger) and "-ful" (meaning full of). The word "thimbleful" refers to a small amount that fills a thimble.
Think of a thimble, which is a small cover for your finger, and remember that a thimbleful is a small amount that fills it.