so i heard this foam my trainer he did it as a joke to someone but i think it could work as a neg opener.

you: eww you smell like up dog

her: whats up dog?

you:ahh not much just chilling, you ?

hopefully get some giggles and also (hopefully being told she smells you would have lowered her self esteem so nagging her i think but im new to this so maybe im wrong)

Bad way to open IMO. Just sounds like a insult. Negs are not insult and they need to be calibrated.

Of course you can pull it off if you have really good energy and vibe from you.

It sounds ok to me to open up groups.

Honestly, I think that sort of opener should be reserved for those of ages 14 and below. It sounds pretty childish in my opinion and using that sort of opener while (very mildly) humorous is pretty poor overall.
I'm not a PUA -yet, just an AFC and I don't seek to further dilute the quality of this forum anymore by adding myself to one of those many pretending to be something they're not. Nevertheless I have watched this forum for over a year. One thing I have picked up is that sexual framing is vital, and whilst being funny is good it isn't sexually stimulating and with that sort of quality it's probably just going to DLV or sound terrible. You'd be better off with something more simple - I think.

Everything I said is by no means concrete, I'm still learning too. I'm very socially active with loads of men and women and a joke like that wouldn't go down well.

Depends on the situation, your looks, your delivery, depends on so many things. In my opinion; you need to be good looking, and have lots of charisma and it will work.... just depends on where you go from there. Yes it will open you and get you chatting, depends on where you go from there... saying that line will not get you into her bed or your bed ....

Why are ppl opening with neg? and why neg to begin with?

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Still learning

This is honestly very bad - it's a cheesy joke that everyone's heard before and makes you sound very unoriginal and boring. Just being honest. Literally, you will have WAY more success with something as simple as "hi".