The word "swinger" is a transparent Modern English compound built from the native verb "swing" and the agentive suffix "-er." The verb "swing" itself has deep Germanic origins, evolving from the Old English swingan, which carried a stronger sense of striking, dashing, or rushing. Over centuries, its meaning softened and specialized to primarily denote a rhythmic, pendulous motion. The addition of the "-er" suffix, a standard feature in English for forming agent nouns (like "runner" or "teacher"), directly creates the meaning "one who swings." This logical construction first described a person engaged in the physical act of swinging, but in the mid-20th century, it acquired a distinctive social slang meaning, applying the concept of "swinging" metaphorically to a lifestyle perceived as moving freely between partners or conventions.

Structure: swing (to move back and forth) + er (agent suffix) swing: From Old English swingan (to beat, fling oneself, rush). A Germanic root indicating a to-and-fro or violent motion. Its functional role is the core action. er: A native English agent noun suffix from Old English -ere, used to form nouns denoting a person or thing that performs the action of the verb. Its functional role is to indicate the agent performing the action of "swing".