In a perfect manner without any mistakes, defects, or error.
Derived from 'faultless' (adjective) plus '-ly' adverbial suffix. This represents two suffixes stacked together: '-less' + '-ly', creating an adverb from an adjective. Both suffixes are ancient in English.
Multi-suffix words like this show how English builds complexity by stacking small units together like Lego blocks! 'Faultlessly' demonstrates that you can apply '-ly' to almost any adjective to create an adverb, which is why English speakers can create adverbs on the fly.
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