In practice, if you're reading a J expression which contains unfamiliar primitives, there's only one place you need to look: https://code.jsoftware.com/wiki/NuVoc

Idioms are composed from primitives, but many of them are shorter than a descriptive name would be, and since the set of primitives is small (compared to the standard library in most languages), it doesn't take that long to memorize most or all of the symbols.