Know those records that you insert into a vinyl record player? Basically those, but pirated since a real one costs money when you purchase them at official music retailers. It involves the process of obtaining a legal copy then using wax molds (to recreate a record) and specialized tools (prevent quality loss) duplicating the record. (Also, “Bone Music” existed in the Soviet Union by printing music by using x-ray films).
Bootleg records were a thing in Europe in the days of reel to reel tape as the only alternative. It wasn’t so much that people did it privately but people would try to make a buck through re-sale and especially on flea markets where oversight by the law was virtually nonexistent. Rare records have always been a thing. Bootleggers tried to profit off it.
I was bequeathed my parents’ record collection of about 200 LPs. One was a bootleg they kept, some rare Beatles stuff. Other ones were thrown away because the quality was bad or would have been deteriorating to a point where it became unlistenable.