Anything that is not a cow
. It is sometimes used for tags when you can't think of anything else to put in. Also, It can be used for answering a question that you don't know the answer to.
1. TAGS: Carrot, pie, elf, lichen, omelette, Not a Cow
2. Manager: "So, Alex, what have you been working on?"
Alex: " Uh.... Not a Cow?"
Manager: "You're fired."