Conversation so far:
What is girl power according to the Spice Girls?
The phrase was a label for the particular facet of post classical neo-feminist empowerment embraced by the band.
What other meaning did the phrase girl power have for the Spice Girls?
The phrase means that a sensual, feminine appearance and equality between the sexes need not be mutually exclusive.

Question: When did them saying girl power become popular?
Self-contained rewrite: When did the saying girl power become popular?