From the Oxford English Dictionary:


girl power, n.

Brit. /gl pa/, U.S. /grl pa()r/ Forms: 19- girl power, girl-power, girlpower, (in sense 2) grrrl-power. [< GIRL n. + POWER n.1 In sense 1, formed in contradistinction to MANPOWER n. In form grrrl power (see sense 2, quot. 19922) after GRRRL n.; cf. RIOT GIRL n.]

1. The number of girls available to perform a task; girls considered collectively in relation to their capacity to perform work. Contrasted with MANPOWER n.
1952 M. LOWRY Let. 24 Nov. (1967) 323 Nearby is a Catholic church within which it says: ‘We want girl-power for our convent.’ 1963 Population Index 29 356 A dowry was required for a girl's marriage. Her farm family..needed cash more than it needed girl-power. 1989 St. Louis (Missouri) Post-Dispatch (Nexis) 18 June 1D, The St. Louis Boy Scout and Girl Scout councils and the Missouri DeMolay, the Masonic youth organization, quickly supplied the boy- and girl power. 1996 News Tribune (Nexis) 10 Mar. C1 Why waste so much manpower (more precisely, boypower and girlpower) in holding dozens of games designed to fill footnotes in a record book?
2. orig. U.S. Power exercised by girls; spec. a self-reliant attitude among girls and young women manifested in ambition, assertiveness, and individualism. Although also used more widely (esp. as a slogan), the term has been particularly and repeatedly associated with popular music; most notably in the early 1990s with the briefly prominent ‘riot girl’ movement in the United States (cf. RIOT GIRL n.); then, in the late 1990s, with the British all-female group The Spice Girls.
1986 Los Angeles Times (Nexis) 16 June V. 1 ‘Girl Power: Health Power’ motivates girls to assume long-term responsibility for their health and fitness. 1992 Star Tribune (Minneapolis) (Nexis) 14 Mar. 1B, Beatrice Joseph, 7, helped dissect a sheep's heart Friday to learn about the heart as part of the Minneapolis YWCA Girl Power program. 1992 Houston Chron. (Nexis) 4 Oct. (Lifestyle section) 3 Places like girl bedrooms, girl bathrooms and punk-rock clubs such as 924 Gilman in Berkeley, Calif., that have specially arranged girl nights, when girl bands sing songs about girl power. 1994 Rolling Stone 16 June 24/4 It has four all-girl bands... All amazing. Part of the grrrl-power underground that's so exciting. 1997 J-17 June 44/3 If something you've bought isn't up to standard, remember you're entitled to complain. Use your girl power and don't let the shop assistant fob you off! 1999 Times 2 Aug. 43/4 This was the fabulous, horrible lie of Girl Power as a movement with absolutely no meaning, it became a disembodied slogan capable of being slapped on any act. Girl Power, it seems, is simply Anything A Girl Does. 2001 Dreamwatch Mar. 24/1 Cameron has done it by mixing the sober feminism of his Terminator and Aliens characters with the sexed-up Girl Power of a Britney Spears concert.

I am fully in favor of taking up the first definition again--"I think we'll need 300 units of girlpower for this one, sir." Hee hee.

The 1999 example could pretty much be directly related to the 1997 example (in part two of the definition).