What peculiar property do all these words have in common?
AMP APP BARF COME CUR EAR GANG GROW IMP ON PEAR PIMP SING STAB TRIP
As @TwoBitOperation astutely observed, it's not just that you can add –LY to each of these words to get a new word. It's that the new word is etymologically distinct from the original word. The original word is not the stem/base of the new word, and sometimes it even changes pronunciation upon adding –LY.
It actually took a fair amount of deep dictionary research to be sure. For example, I didn't realize that HARDLY really does come from HARD, and HOMELY really does come from HOME. I'm still not sure about PORT/PORTLY, as I found the etymologies confusing.
A few others that didn't make the shortlist: