The American Heritage Dictionary of the English Language, fifth edition (2010) has this entry for loquacious:
loquacious adj. Very talkative; garrulous [which AHDEL defines as "given to excessive and often trivial or rambling talk; tiresomely talkative" or "wordy and rambling"].
Merriam-Webster's Eleventh Collegiate Dictionary (2003) has this entry:
loquacious adj ... 1663 1 : full of excessive talk : WORDY [which MW defines as "using and containing many and usu. too many words" or of or relating to words : VERBAL"] 2 : given to fluent or excessive talk : GARRULOUS [which MW defines as "given to prosy, rambling, or tedious loquacity : pointlessly or annoyingly talkative" or "WORDY 1"]
However, when it attempts to distinguish loquacious from talkative, garrulous, and voluble, the Eleventh Collegiate offers a rather surprisingly positive take on loquacious. Here is the entire usage note (which appears at the end of the entry for talkative):
TALKATIVE, LOQUACIOUS, GARRULOUS, VOLUBLE mean given to talk or talking. TALKATIVE may imply a readiness to engage in talk or a disposition to enjoy conversation {a talkative neighbor}. LOQUACIOUS suggests the power of expressing oneself articulately, fluently, or glibly {a loquacious spokesperson}. GARRULOUS implies prosy, rambling, or tedious loquacity {garrulous traveling companions}. VOLUBLE suggests a free, easy, and unending loquacity {a voluble raconteur}.
To the extent that loquacious "suggests the power of expressing oneself articulately, fluently, and glibly," it would seem to have very little overlap with "prosy, rambling, or tedious loquacity"—or in other words, with garrulous. The central notion of loquacious in the usage note involves command and aptness of language to maximum effect, while the central notion of garrulous is endless, boring prolixity. It almost seems as though the MW definition of loquacious in the main entry and the definition in the usage note come from different eras that had different understandings of the word's meaning. But the odd split in definitions goes back more than half a century. Here is the entry for loquacious in Webster's Fifth Collegiate Dictionary (1936):
loquacious adj. ... Given to talking; garrulous [which the same dictionary defines as "talking much; also, wordy; diffuse"]
And here is the description of loquacious in Webster's Dictionary of Synonyms (1942):
Loquacious and loquacity more commonly imply fluency and ease in speech: they may variously imply qualities as diverse as the power of expressing oneself articulately or eloquently or a disposition to indulge in chattiness or glibness.
The updated Merriam-Webster's Dictionary of Synonyms (1984), however, cuts out the allusions to articulate or eloquent expression:
Loquacious and loquacity more commonly imply fluency and ease in speech or, sometimes, an undue talkativeness.
What happened to the high-end senses of loquacity? I don't know—but somehow the notion of articulate expression has managed to persist in Merriam-Webster's usage note for talkative and its allies through multiple editions of the Collegiate Dictionary, long after it vanished from the Dictionary of Synonyms.
Two other dictionaries of synonyms offer their own interpretations of loquacious. James Fernald, English Synonyms and Antonyms, thirty-first edition (1914) offers this comparison of talkative, loquacious, and garrulous:
The talkative person has a strong disposition to talk, with or without an abundance of words, or many ideas; the loquacious person has an abundant flow of language and much to say on any subject suggested; either may be lively and for a time entertaining; the garrulous person is tedious, repetitive, petty, and self-absorbed.
And S.I. Hayakawa, Choose the Right Word: A Modern Guide to Synonyms (1968) offers this comparison of loquacious and garrulous:
Unlike the previous pair [voluble and glib], loquacious and garrulous are less concerned with verbal smoothness than with talk that is incessant or lengthy. Of the two, loquacious is less clear-cut in tone. When positive, it serves as an intensification of the favorable implications of talkative, with a suggestion of the fluency implicit in voluble: an earnest and loquacious advocate of open housing. When negative in tone, however, it can suggest an overbearing insistence on holding forth without regard for one's listeners: loquacious guides who will not let you look at a masterpiece in silence; a loquacious public speaker who always exceeded by half the time allotted for his speech. Garrulous is wholly negative in tone and as such is an intensification of the negative possibilities in loquacious.
Hayakawa's description of loquacious comes closest to my understanding of the variable sense of the term—but whether that reflects the continuing accuracy of his description or merely shows that I am stuck in the late 1960s, I cannot say.
