std is an abbreviation for standard.
When used in this sense, it is fun to play with new C coders' heads (and normal people) by talking about stdio and stdlib when they only know about the other meaning.

The other common meaning is sexually transmitted disease, in which case the abbreviation is STD.

In the book Snow Crash by Neal Stephenson, STD stands for a severe tire damage device; that is, something in the road surface that ruins your tires should you try to drive over it. Outside the book, when used alone (that is, without the "device" part), it states what will happen to your tires when you drive over one in the wrong direction.