Neg means negative compliment.
Say for example the girl dressed in tight jeans. You can say :"Very nice outfit, altough I prefer skirts". You compliment on her dress, but you also make it clear, that it doesn't impress you. A neg is often confused with an insult, altough in reality it is really a compliment.
A neg can have different uses. Most importantly it keeps your target wondering whether you like her or not. The key is that she must feel that you like her but mustn't know for sure until it's about time
But a neg can also be used to flirt. For example: "I really like your dress...wonder what is underneath. Oh I meant your personality"
It can help you lead the frame: "I have a thing for adventurous girls. You aren't one of them are you?" (She will most probably try proving you wrong and you can start talking about fun stories)
Hope this helped you!
