means Cooked by dry heat in an oven or over fire until browned, or—in slang—mocked relentlessly and cleverly to someone's face.

from From Old French 'rostir,' to roast over a fire, which English borrowed in the medieval period and which traces back to a Germanic root for grilling on a grate. The 'mocking' sense is recent slang, built on the old image of holding something over flames—roast someone and they get the heat.