A bag containing sweets and a cheap toy or gift, as bought by children from sweetie shops. The point is, the purchaser doesn’t know exactly what is in the bag until it is opened, and the term is often used to disparage something the speaker doesn’t think much of:
“That’s some mobile phone you’ve got there; get it in a lucky bag, did ya?”
* I suppose the American term for this is Crackerjack box.
Hear my audio.