It is said to be a combination of two popular brands of rolling paper, Zig-Zag and Top or as a tribute to blues legend Z. Z. Hill. It actually came from blues guitar master B. B. King and while they wanted to call themselves Z.Z. King, they felt it was too close of a name. So, since B.B. was at the top in the recording industry, ZZ Top was the final decision.
https://en.wikipedia.org/wiki/ZZ_Top