fillip
The action of holding the tip of a finger against the thumb and then releasing it with a snap; a flick.
archaic
A sharp strike or tap made using this action, or (by extension) by other means.
Arch[ers]. In default of six pistoles, / Choose then without ado / To receive thirty fillips, / Or twelve blows with the stick. / Punch. If it must be, and that I must pass through that, I choose the fillips.
Something that excites or stimulates.
This measure gave a fillip to the housing market.
Where it comes from
From Middle English filippen, philippe (“to flick or snap a finger from against the thumb”); further origin uncertain, but probably imitative. See flip. It is not clear whether the verb is derived from the noun, or vice versa.
Adapted from the Wiktionary entry fillip (CC BY-SA 4.0).