In a reliable and trustworthy manner; in a way that can be counted on.
From dependable plus -ly (Old English -lice, adverbial suffix). The formation shows how English converts adjectives to adverbs by adding -ly, a pattern that became standardized during Middle English.
The adverb 'dependably' is what you'd use to describe how a clock works or how a good friend acts—it's about performance and behavior, not just potential, which is why you rarely hear 'potentially dependably' because that would be contradictory.
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