Yesterday, I finished writing an article on Lionel Deimel’s Farrago about adjectives ending in ly, that is adjectives that look like adverbs. I became interested in such words when I was made aware that “friendly” is an adjective (“the friendly neighbor”), but not an adverb (not “the neighbor greeted us friendly”). Using my imagination, and with the help of Google, I was able to identify about 150 adjectives ending in ly. Some of these words are also adverbs, e.g., “weekly.” Some, arguably, have no adverbial form, but authors disagree about which ones are in this category. (My favorite is “thistly.” It is difficult, though perhaps not impossible, to imagine a use for an adverbial form of this word.) Then there are words like “friendly,” which form admittedly rare adverbs by replacing ly with lily. Not everyone acknowledges that “friendlily” is a real word, but it is in dictionaries. One can certainly imagine uses for the word—“all the neighbors behaved friendlily”—but its use is easily avoided (e.g., “all the neighbors behaved in a friendly manner”).
Decide for yourself which adjectives ending in ly have useful adverbs ending in lily by reading “Odd Adjectives.”