In a manner lacking faith, trust, or loyalty; without belief or faithful commitment.
From 'faithless' (faith + -less suffix, Old English, meaning without) plus '-ly' adverbial suffix. Double suffix formation creating manner adverbs.
The '-less' suffix is Old English's way of negation—stacking '-less' then '-ly' creates layered negation, making 'faithlessly' almost sound like it has no faith to begin with!
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