The medicine was so bitter that he could only take one spoonful at a time.

spoonful 是由名词 spoon（勺子）加上后缀 -ful 构成的名词，表示"一勺的量"。后缀 -ful 常用来表示"充满...的量"，如 cupful（一杯的量）、handful（一把的量）等。