A complaint on social media the other day about people who use reticent to mean reluctant called the usage an error and a recent corruption of the language.
It is not necessarily an error, and it is not all that recent.
The traditional meaning of reticent is “inclined to be silent” or “restrained in expression.” Taciturn is an apt synonym.
But Merriam-Webster’s Dictionary of English Usage identifies the “reluctant” sense as having been current in the United States since the early 1950s, and steadily gaining ground since. The sense of “hesitant” or “reluctant” has expanded far beyond the original association with speech, MWDEU notes.
The Corpus of Contemporary American English does have citations in which speech is the context, but the abundance of “reticent to” examples suggests that the newer sense is swamping the older.
The fourth edition of Fowler’s Dictionary of Modern English Usage suggests that there is “an air of inevitable semantic shift” about the usage. And in Garner’s Modern American English, Bryan Garner concedes that in some contexts “the difference between taciturnity and reluctance is extremely subtle.” Though he still considers the “reluctant” sense a misuse, he rates it at Stage 4 “Ubiquitous but …” in his language change index.
Semantic shift, like continental drift and evolution, happens, sometimes quickly, sometimes slowly, and you have to decide the point at which resistance to it ceases to make sense.