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The word “bad” (c. 1300) has an uncertain, likely Germanic origin, possibly stemming from the Old English bæddel (“hermaphrodite, effeminate man”) or related to bædan (“to defile”). It replaced evil as the common opposite of “good” around 1700 and historically meant worthless, wicked, or counterfeit. Online Etymology Dictionary +3

Key Aspects of “Bad” Etymology:

• Uncertain Origins: The origin is debated and not definitively traced, appearing late in Middle English (13th-14th century).
• Potential Source: Many etymologists suggest it derives from the Old English derogatory term bæddel or bædling, which meant “effeminate man,” “hermaphrodite,” or “pederast”.
• Alternative Roots: It may be related to the Old English bædan (“to defile”) or, as suggested by Wiktionary, related to Old Norse/Norwegian terms for “damage, effort, or trouble”.
• Phonetic Evolution: The transition from bæddel to bad is phonetically possible but not universally accepted as direct, per Merriam-Webster.
• Semantic Shift: Originally, it meant inadequate or vicious, but by the 1890s, it developed an ironic slang meaning of “good” or “impressive,” particularly in Black American slang.
• Debased Coinage: It was frequently used to describe counterfeit or “debased” money, notes this Reddit post. 

*Note: The German word “Bad” (bath) is unrelated, originating from a different Germanic root (baþą), as explained on Wiktionary.