Coined by
South Park's Jimbo and Ned, it can be observed in the episode "The Mexican Staring Frog of Sri Lanka" - ep6 in 2nd season. It's in the "Huntin' and Killin'," Jimbo's and Ned's show about hunting and killing.
Pussy law #4 states that hunters are not permitted to
kill animals in self-defense, thus yelling the line "It's coming right for us" is no longer usable to legalize the animal murder.
For that reason, Jimbo and Ned have to yell "Thin out their numbers" to "legally"
kill the animals.
Jimbo: "Now, idn't that great? We have a terrific show for you today: we're gonna
kill some elk, and we're gonna
kill some mountain goats. Now, the new law passed by Colorado legislature, which Ned and I call 'Pussy Law #4', states that we can no longer
kill animals in defense. In other words, our old line of 'It's comin' right for
us' no longer works. So now, we only
kill naimals to, quote, thin out their numbers. If we don't hunt, then these animals will grow too big in number and they won't have enough food. So you see, we have to kill animals, or else they'll DIE. "