The verb 'wag' emerged in Middle English (c. 1300) as 'waggen', meaning to shake or move back and forth. Its origin is likely onomatopoeic or expressive, imitating the motion it describes, and it is related to other Middle English words like 'waggle'. It belongs to a family of Germanic words suggesting restless motion, such as Old Norse 'vagga' (cradle) and Middle Low German 'waggen' (to rock). The core concept of rapid, short, oscillating movement has remained consistent from its imitative roots to its modern usage, most iconically applied to a dog's friendly tail movement. The '-ing' suffix follows the standard Germanic pattern for creating verbal nouns and adjectives from the base verb.

The word 'wagging' is the present participle or gerund form of the verb 'wag'. The root 'wag' is considered a simplex word of Middle English origin, likely imitative in nature. The suffix '-ing' is a productive English suffix of Germanic origin used to form present participles, gerunds, and verbal nouns.