To "goose" someone means to poke or pinch them, usually on or between the buttocks, as a prank or playful gesture. It is often done to startle or tease the person. This slang usage of "goose" can also mean to touch or press someone's bottom quickly, causing surprise or laughter. Additionally, "goose" can be used informally to mean encouraging someone to take action or revving an engine, but in the context of "goosing" a person, it specifically refers to the prank of poking their buttocks.
In some contexts, "goosing" might be directed at the sensitive area where the buttock meets the thigh, and it is typically done as a lighthearted joke among friends.