Audacious is one of those words whose splendor consists, at least in part, of its representation of two diametrically opposed meanings. Most of the information herein is from the incomparable Oxford English Dictionary; Merriam-Webster is a useful adjunct to the OED, not a substitute. It may be characterized (as it is audaciously on its website) as "America's most trusted online dictionary;" the OED is English.
On the one hand, daring, bold, confident, intrepid, as in T. Nicolls' translation of Thucydides' History of the Peloponnesian War ii. cvi. 67: "More bolde and audacious in this thing, wherein we have much experyence."
On the other, unrestrained by or expressing defiance of the principles of decorum and morality; presumptuously wicked, impudent, shameless, rash, insolent, as in Daniel Defoe's Moll Flanders (1722), 323: "I grew more hardn'd and audacious than ever."
Merriam-Webster has a helpful descriptive paragraph:
"Audacious first appeared in English in the mid-1500s. It was borrowed from the Middle French adjective audacieux, which was derived from the noun audace ('boldness, audacity'). Audace came from the Latin audacia, a derivative of the Latin root audac- ('bold'). Audac- is also the source of audacity, which appeared in Middle English (as audacite) in the 1400s. Audac- can be traced, by way of the Latin verb audēre ('to dare'), to the Latin adjective avidus ('eager' or 'greedy'), which was also borrowed by English, either directly from Latin or via the French avide, to give us our adjective avid. Among the early adopters of audacious was William Shakespeare, who used the word seven times in his plays, as in Henry VI, Part 2, where Somerset addresses York with the lines, 'I arrest thee, York, / Of capital treason 'gainst the King and crown. / Obey, audacious traitor, kneel for grace."
I promise to toast here in Reckonings any reader who persuasively reveals any other word in English that similarly conveys two such opposing meanings.