The usual term is simply "negative g", but it is implied that the g is gravity. I'm not sure of the earliest mention, but I did turn up this. A Decade of Basi

The usual term is simply "negative g", but it is implied that the g is gravity. I'm not sure of the earliest mention, but I did turn up this. A Decade of Basic and Applied Science in the Navy From 1958. https://books.google.com.au/books?id=QbDyJ8jM7_kC&pg=PA101&lpg=PA101&dq=earliest+mention+of+%22negative+g%22&source=bl&ots=ete6nrI8gK&sig=ACfU3U32Rwr_RBj7MkUAOxJuZ2oDfo27Ww&hl=en&sa=X&ved=2ahUKEwjmiq_C6dfwAhUM4XMBHbt3BsEQ6AEwEXoECBYQAg#v=onepage&q=earliest%20mention%20of%20%22negative%20g%22&f=false https://books.google.com.au/books?id=QbDyJ8jM7_kC&pg=PA101&l...