Traditional grammarians going all the way back to Donatus are accused of classifying as adverb any word they couldn't make fit anywhere else in the canonical parts of speech.
It's a very old criticism. The Stoic grammarians are suspected of having their tongues skewed into their cheeks when they employed the word pandektes ('all-receiver') for adverb. John Horne Tooke, The Diversions of Purley, 1786, called the adverb “The common sink and repository of all heterogeneous and unknown corruptions”, citing the 4th- to 5th-century grammarian Maurus Servius Honoratus:
— Omnis pars orationis, every word, quando desinit esse quod est, when a Grammarian knows not what to make of it, migrat in Adverbium, he calls an Adverb.
That’s a joke (what Servius actually says is that every part of speech can be converted into an adverb); but Horne Tooke was one of the first to recognize explicitly that the term adverb doesn’t really tell you much about how a word so designated actually functions syntactically.
Contemporary grammar has of course progressed beyond the classical notion; we no longer call unclassifiable terms adverbs but employ the far more meaningful term particle.
