Lululemon does not mean anything in slang.
Lululemon is just a brand name that was chosen intentionally for having many "L's to appeal to the Japanese market by making the brand name sound exotic.
Although some people say that the founder of lululemon found the word lululemon amusing to hear Japanese speakers try to pronounce the word due to the L sound's which are difficult for the Japanese people to say.
The name lululemon itself does not have any inherent meaning, but was simply a strategic choice of branding for it's founder.