A stranger DM'd me to hang out; I just replied 'not meeting'.
Tips
culture
Originated around 2015 as a dismissive reply to strangers asking to meet up (often with sexual undertones on dating apps). The full expression was 叔叔我们不约 (uncle, we're not meeting up), now abbreviated to just 不约.
register
Casual and a bit jokey. Between friends it's simply 'not up for it'; online it can carry the older flirty-refusal flavor, so read context.