To be infertile and unlikely to impregnate a woman by having sex with her (from a term used to describe the firing a gun loaded with bullets designed not do harm to the target)
Blanks could be "munição de festim", used in exercises or in movies special effects. As the range and the powder is so dispersed and not much, it is not harmful at a certain range (that is, very close range - "à queima-roupa).
Perhaps from that came the expression point-blank range, a shot that is very close (even if the shot is with live munition) meaning that someone fired a shot at very close range."I thought they were firing blanks," says Canfora. But just as he ducked behind a tree a bullet tore into his right wrist.http://articles.latimes.com/1990-05-02/ ... nt-state/2"A student came up to me and said, 'They're shooting blanks,' " Lewis recalls. "I said no, they're real bullets. Then I indicated to him Sandra Scheuer who was lying on the ground nearby and who I later learned was dead."http://articles.latimes.com/1990-05-02/ ... nt-state/2http://en.wikipedia.org/wiki/Blank_%28cartridge%29
The imagery of "shooting blank" can be used also in the the soccer/football world. In the sense of "failing", when everyone expected goals, and the guy has the opportunities but fails to score.Luis Figo, Cristiano Ronaldo and the rest of the Portuguese attack could only fire blanks on an evening that became increasingly frustrating for them.