1
[ adverb ] in a believable manner

Examples

"he acted believably sincere"

Used in print

(Arthur Miller, "The Prophecy," in The Best...)

Stowey_Rummel was internationally famous , a crafter of a genuine Americana in foreign eyes , an original designer whose inventive childishness with steel and concrete was made even more believably sincere by his personality .

2
[ adverb ] easy to believe on the basis of available evidence

Examples

"he talked plausibly before the committee" "he will probably win the election"

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