Means overly sentimental, soft, or a bit weepy in a harmless way. If you’re being soppy, you’re getting all mushy over something like a film, a love song, a dog advert, or your mate’s engagement post. Can be affectionate teasing rather than a proper insult. Honestly, everyone’s soppy when nobody’s watching.
"We stuck on that John Lewis advert and Dave’s there sniffing like he’s been dumped. I go, Don’t be so soppy, mate. He goes, Shut up, the penguins found love."