English gloss: (noun) the secret exchanging of emails
assembled from: koso+ado
koso is from こっそり (kossori, stealthily)
ado is from メールアドレス (mēruadoresu, email)
So kosoado is exchanging phone emails on the sly. This usually happens at gōkon (singles mach-up parties), when you and your target have stolen away for a second of privacy. Another sound for stealth that also comes from kossori is こそこそ (kosokoso, the mimetic word for sneaking). It's a pretty humorous word to me, but then all gitaigo are.
Being a foreigner, I've never been invited to a gōkon myself. That would be a disaster: either everyone would focus on me and my amazing foreignness, or they would ignore me as they started talking fast, slangy Japanese and assumed I can't keep up. "No need for kosoado, the foreigner will never notice if we exchange mails right in front of him!"
So I bet you are wondering what a gōkon is. The following vid is an very accurate presentation of one. Only they exchange their mails openly.
And here is one with more party games: