'baddest' originates from Modern English formation: the adjective 'bad' plus the superlative suffix '-est' (English), where 'bad' in general meant 'not good' or 'of poor quality'.

'bad' in Middle English appeared as 'bad' or 'badde'; earlier influences are uncertain but the word may trace to Old English forms such as 'bæddel' (recorded meaning 'effeminate man' or 'hermaphrodite') or other Germanic sources. Over time the Middle English forms developed into modern English 'bad', and the superlative 'baddest' arose by regular adjectival comparison (bad → badder → baddest).

Initially some early senses related to weakness or effeminacy in medieval usage, but the dominant meaning shifted to 'not good; poor in quality' in later Middle English; in modern usage the word expanded, and in colloquial/slang contexts it can also mean 'extremely impressive' or 'formidable'.