From Wikipedofilia:
"The band's name was rumored to have derived from Zig-Zag and TOP rolling papers. Gibbons, however, revealed the true origin of the group's name in his autobiographical book Billy F Gibbons: Rock + Roll Gearhead. The book mentions about an apartment that Gibbons lived in, with a row of flyers on a wall. Taking notice of Z. Z. Hill and B.B. King posters, Gibbons favored "ZZ" and "King," and came up with "ZZ King," though it was too much like the guitarist's name. Coming to the conclusion that B.B. King was on the "top," Gibbons settled with the name "ZZ Top."
So, there ya go...