What does “杀牛” mean in this context? In the story I'm reading for class, a man meets a woman online and she asks him what sign he's born under (属什么). He says 牛, and she says "我是杀牛的." He recounts, "就这么两句话一下子就把我打动了." So they arrange to meet in the park and it turns out she's old and gray instead of young and pretty like he expected, and he thinks "直到这个时候我才什么都明白，眼前的这个女人为什么是“杀牛的”了."

What on earth is going on? “杀牛” has to have some idiomatic or slang meaning, right?
(From the view of the man), the 1st 杀牛的 means men born under the Ox sign killer, the 2nd one means (real) Ox killer.