No way! / Not a chance!
'Manco' — MAN-co. Stress on the first syllable. Used as 'not even' in dialects, absorbed into standard colloquial Italian.
Use with close friends to vehemently refuse or disagree — more colourful and emphatic than 'no'. Particularly expressive of surprised refusal.
'Manco' is a colloquial Italian word meaning 'not even' (from 'nemmeno' / 'neanche'). 'Manco per idea' (not even for the idea) is an idiomatic way to refuse an idea outright — 'not even the thought of it!'
Ni morti!
Absolutely not!
Southern Italian slang — literally 'not dead' — means 'absolutely not'. Vivid.
Ma quando mai!
When has that ever been! / No way!
'Ma quando mai' — rhetorical question implying something has never happened and never will
Nemmeno se mi pagassero.
Not even if they paid me.
Conditional refusal — 'pagassero' is congiuntivo of 'pagare'. No amount of money would change your mind.
'Torto marcio' (rotten wrong — completely and inexcusably wrong) is a vivid Italian intensifier. The '-marcio' (rotten) suffix adds moral weight to the wrong — not just incorrect but deeply, obviously wrong. A staple of Italian domestic and political arguments.