"Promiscuous" is in fact a now dated implied synonym for "fast": "fast" in the sense of "quick or very open to engaging in sexual activity". You're right about

"Promiscuous" is in fact a now dated implied synonym for "fast": "fast" in the sense of "quick or very open to engaging in sexual activity". You're right about many of the rest not being synonyms per se, although they are related words (related to "religious fasting" for example).