isn't "zero-cost abstraction" shorthand for "zero-performance-cost abstraction"? i.e. computer, not human, performance?

It is! Which is why I don’t like the term. It often masks the fact that it has a cost in complexity.

i've never read it that way! could you give an example of that?

Two examples in C++: virtual method dispatch via vtable (in C you'd implement the same mechanism manually), templated generic code. C++'s generic data structures and algorithms are faster than C's because they don't have to use indirection through pointers. The compiler creates specialized code for each type instead. It causes binary size bloat, but execution time is low.

so what's the "cost in complexity" here? the complexity would be there whether you went with a a hand-rolled or "zero-cost" version (at least that's the idea), except the "zero-cost" version should require less code and thus be easier to maintain.