Final answer:
In the given context, 'alacrity' means briskness, which refers to a quick and eager readiness.
Step-by-step explanation:
In the given context, the word 'alacrity' means briskness.
Alacrity is a noun that describes a quick and eager readiness to do something. In this case, the person wearing the uniform jacket quickly raised his weapon to his shoulder upon seeing a white man on the path. This action shows the person's alertness and vigor, indicating the meaning of briskness.