The word "赴汤蹈火" is formed by combining two parallel verb-object constructions: "赴汤" (to go into boiling water) and "蹈火" (to tread into fire), where 赴 means "to go to", 汤 means "boiling water", 蹈 means "to tread on", and 火 means "fire". This structure directly juxtaposes two hazardous scenarios to create a compound idiom expressing the act of facing extreme dangers.