I grew up in Malaysia and Singapore, and it's taken me a long time to dissect my vocabulary into "local slang, incomprehensible/incorrect elsewhere" and "proper English".
'Purposely' is one of those words that's just felt wrong to me for a while. It's an adverb of the + 'ly' form, except that "purpose" isn't an adjective. However, I've seen the word used so often on the Internet I can't help but feel that it's not just us South-East Asians doing it.
Is "purposely" grammatically correct? If so, in what context? If not, what is an appropriate substitute, and why is it so common?