Green’s Dictionary of Slang

1. permissive.

1894		Mirror of Life 17 Feb. 2/3: So long as he can keep a cliental which is profitable, he leads a ‘rapid’ existence,.
1907		Harrington & Tate [perf. Marie Lloyd] ‘Customs of the Country’ 🎵 Paris is a rapid place, it wouldn't suit your wife.

2. (Irish) a general term of approval, excellent, wonderful.

1988		R. Doyle Commitments 45: You’re Declan Blanketman Cuffe. —That’s a rapid name, said Outspan.
1991		F. Mac Anna Last of the High Kings 107: I’ll borrow me Pa’s Jag. We’ll have a rapid time.
2001		G. Coughlan Everyday Eng. and Sl. 🌐 Rapid (a): amazing.