In Mark 2 and other places we're told that Jesus ate with tax collectors and "sinners." I've always wondered, who were these "sinners" and what exactly did they do to earn such a title? Has anybody ever looked into that?

I think most of the population would fit in there. Remember, there were may translations and interpretations of the words from Hebrew, Aramaic and Greek. Jesus told the woman at the well, "Go and sin no more". Aren't you glad you didn't live back there? Here are a couple from the OT!!

God ordered death for the following sins and people.
1. Murder (Ex 21:12,14)(Lev 24:17,21)(Num 35:16-21,30-31)
2. Kidnapping (Ex 21:16)(Deut 24:7)
3. Child sacrifice (Lev 20:2)
4. Both the man and woman who commit adultery (Lev 20:10)(Deut 22:22-24)
5. Rape (Deut 22:25)
6. Daughter of a priest who became a prostitute (Lev 21:9)
7. An idolater (Ex 22:20)(Deut 17:2-5)(Num 25:1-5)
8. Breaking the Sabbath (Ex 31:14)(Ex 35:2)(Num 15:32-36)
9. A woman having sex before marriage (Deut 22:21-22)
10. Homosexuality (Lev 20:13)
11. A man and his father's wife who have sex (Lev 20:11)
12. A man and daughter-in-law who have sex (Lev 20:12)
13. A man who marries a woman and her mother (all 3 must die) (Lev 20:14)
14. Bestiality (Sex with an animal) (Ex 22:19)(Lev 20:15-16)
15. A false prophet (Deut 13:5)(Deut 18:20)
16. A false witness (Deut 19:16-21)
17. A disobedient son (Deut 21:18-21)
18. A child who strikes his father or mother (Ex 21:15)
19. A child who curses his father or mother (Ex 21:17)(Lev 20:9)
20. Men who are fighting and hit a pregnant woman, causing her lose her baby (Ex 21:22-25)
21. A man whose ox kills someone after previously goring other people (Ex 21:28-29)
22. A sorceress (Ex 22:18)
23. A medium or spiritist (Lev 20:27)
24. A brother, son, daughter, wife, or friend who entices you to go after other gods (Deut 13:6-11)
25. Everyone in any town that entices people to go after other gods (Deut 13:12-15)
26. A blasphemer (Lev 24:10-16,23)
27. Anyone who failed to abide by a decision of the court (Deut 17:8-12)
28. Any non-Levite who tried to set up or take down the Tabernacle (Num 1:51)

Matthew (Levi) had a dinner party with his guests being Jesus and probably his tax-collector buddies. After some concordancing, I've decided "tax collectors and sinners" is synonymous with "tax collectors who are sinners" (as compared to "scribes and Pharisees who are strict adherents of the Law" -- "the righteous"). As Jesus said, "I have come not to call the righteous, but sinners to repentance."

It seems that every culture comes to regard some sins as acceptable and others as outrageous, so I suppose the sinners in that case were those engaged in the "outrageous".

A good article on this. Of course in General no bible scholars agree on exactly who this was, but this is a good article on it.

https://www.bibleodyssey.org/en/peop...rs-and-sinners

@Fr_Chuck That was a wonderful specific answer. And I second the motion.