A quick flick with a finger against the thumb, or anything that provides a sudden boost or stimulus to something.
Likely imitative in origin, possibly from 'flip,' describing the sound and motion of the finger-flick gesture. The figurative meaning (a stimulus or boost) developed from the idea of a quick, energetic motion.
A fillip to the economy means a sudden boost—the word stuck around from the 1500s even though people don't actually do finger-flicks much anymore, showing how vivid metaphors survive for centuries.
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