I started trying to think of other adjectives that end in "-ly". Alls I could come up with off the top of my head was "friendly" and "miserly", but then I realized that I had my two buddies, grep and /usr/share/dict/words, available to ask. Turns out there are a handful of nouns that end in -ly (pronounced unstressed and phonemically the same as the adverbial suffix; I'm not considering things like "fly"), such as "anomaly" and "assembly", and a heck of a lot of adjectives. I checked the OED just now, and it turns out I came to some of the same conclusions as them. The adjective-forming -ly (which comes, as I remembered correctly, from an old german root, with a modern german reflex of -lich, meaning originally "body" or "form") is used a lot in a eulogizing sense ("brotherly", "comely", "comradely", "courtly", "gentlemanly", "godly", "kingly", "manly", "masterly", "matronly", "neighborly", "queenly", "saintly", "scholarly", "seemly", "sisterly", "soldierly", "sprightly", "stately", "steely") also in a dyslogizing sense ("beastly", "beggarly", "brambly", "costly", "crumbly", "deathly", "earthly", "fleshly", "ghastly", "ghostly", "gravelly", "grisly", "grizzly", "oily", "prickly", "rumply", "scaly", "sickly", "silly", "smelly", "surly" (from "sir" + "-ly"!), "ugly", "wily", "worldly", "woolly")
And after that there's still more: "bodily" "bubbly" "churchly" "costly" "cowardly" "cuddly" "curly" "deadly" "melancholy" "otherworldly" "pearly" "portly" "unlikely" "unruly" "untimely"
The article claims that "gingerly" might be unusual, or even unique, (in the minds of those that do think of it as an adverb and don't think of "ginger" as an adjective) in being an -ly adverb of manner not derived from an ajective stem.
I submit, tentatively, to the contrary:
- haply - although it might be too obsolete to really count. "hap" is just as obsolete, but it's a noun.
- leisurely - I wonder about, since I don't think of it as an adverb, but the dictionary says it can be.
- namely - I don't think of name as an adjective, though OED says it is.
- overly - OED says it derives from the adverb form of "over", even though it does also list an adjective form of "over", which is ungrammatical to my ear.
- purposely - noun root.
- partly - OED says the same thing, though I hear "part" as an adjective just fine. I wonder if that could have been a backformation?
- thusly - an adverb derived from a non-ly adverb, by adding -ly. Crazy! No wonder it's considered sketchy english.
- verily - like haply, kind of obsolete itself, but certainly the adjective sense of "very" seems even less current.
There's a separate issue of whether dropping the -ly necessarily produces a legitimate right away, even though the adverb is clearly adjective-derived. I would say "domestically" but never "domestical", "enthusiastically" but never "enthusiastic". On the other hand "comic" and "comical" are both okay, and "economic" and "economical", but mean slightly different things. And I must say "logical" for the adjective, not "logic", "typical" not "typic", "biblical" not "biblic", "cynical" not "cynic", (and here's a fine example for the topic at hand!) "lexical" not "lexic".
The word "especially" is fine to my ear, but "especial" sounds like it has cobwebs on it. British cobwebs at that.
Also there's a whole series of timed-recurrence-related adjectives and adverbs that are nouns once you drop the -ly: "hourly", "daily", "weekly", "fortnightly", "monthly", "quarterly", "yearly". But "seasonal", "annual", "centennial", are normal in having separate adjective and ly-adding adverb forms.