To masturbate or to be masturbated or to be given a blow job. As heard in the Katy Perry song, "California Gurls".
In some kid friendly performances, she will say "melt" i.e.: the today show, where it is obvious what she is saying.
However, in other performances i.e.:SNL, it will be more obvious that she is actually saying "milk" and will accompany the line with a hand jerking motion.
In the video she pops her cheek to suggest a blow-job.