@anika_voin wiki сообщает:
"The original line-up was formed in Houston and consisted of Gibbons, bassist/organist Lanier Greig, and drummer Dan Mitchell. The name of the band was Gibbons' idea. The band had a small apartment covered with concert posters and he noticed that many performers' names used initials. Gibbons particularly noticed B.B. King and Z. Z. Hill and thought of combining the two into "ZZ King", but considered it too similar to the original name. He then figured that "king is at the top" which gave him the idea of naming the band "ZZ Top"

То есть, объединили прикольные инициалы с фамилией, а затем фамилию Кинг заменили на "топ", т.к. король - он по-любому на вершине. )