I’ve heard this word a lot since I’ve been in Iran. I was talking about my uncle and suddenly he called. Che halāl zādeh!, my cousin exclaimed. Another time, we were talking about a colleague, and he happened to show up out of nowhere. Che halāl zādeh!
As you may have guessed by now, حلال زاده halāl zādeh is like the English, speak of the devil. It roughly literally translates as legitimate child, so when you say che halāl zādeh, it’s like what a legitimate child!/speak of the devil! You can also say che halāl zādeh bud, He/she was such a legitimate child!
Persian uses legitimate children; English uses devils. Interesting.