Green’s Dictionary of Slang

1. (orig. US) extremely sophisticated.

1994		(con. 1967) T. Wolff In Pharoah’s Army 142: They always had friends in the house, journalists, visitors from the States, cryptic young officers from up-country [...] It was like an ultracool fraternity.
2000		Guardian 3 Dec. 🌐 His deceptively low-key, ultracool manner leaves spectators more amazed than if he had bombarded them with special effects.

2. fine, first-rate, problem-free.

2001

W.D. Myers Bad Boy 138: I wanted to hear a big voice on the phone say, ‘Yea, verily, this is me, God. It’s all good, my man, and will be ultracool in the end. Don’t worry.