The word 'tailgater' is a modern American English compound formed in the mid-20th century from the verb 'tailgate'. The verb itself derives from the noun 'tailgate', originally referring to the hinged gate at the rear of a wagon or truck. The semantic shift from a physical object to a driving behavior metaphorically captures the image of one vehicle being as close to another as its own tailgate. The addition of the agent suffix '-er' follows a standard English pattern, creating a noun that explicitly denotes the person performing the action of following too closely. Thus, the term logically builds from concrete parts to describe a specific and potentially hazardous social behavior on the road.

Structure: tailgate (to follow too closely) + er (agent suffix) tailgate: A compound word from English 'tail' (the rear part) + 'gate' (a movable barrier). In this context, it functions as a verb root meaning to drive dangerously close behind another vehicle. er: An English agent suffix of Germanic origin, used to form nouns indicating a person or thing that performs the action of the verb.