恩恩 (also written
嗯嗯, the more standard form) is internet/texting slang. It's softer and warmer than a single
嗯, but in dating/relationship contexts it can read as lukewarm or dismissive - "I read your message but don't have much to say".
恩 properly means "kindness / favor"; the use here is purely phonetic, borrowing
恩 because typing it is faster than
嗯 on some IMEs.
