* It's a misspelling: Maybe you meant "Natsu-cow," which could be a slang term for something related to the anime character Natsu Dragneel from Fairy Tail. However, there's no official use of this term.
* It's a specific regional dialect: There might be a regional dialect or slang where "Nat Su Cow" means something specific. If you could provide more context about where you encountered this phrase, it might be easier to identify its meaning.
* It's a made-up term: It's possible that "Nat Su Cow" is a made-up term or a joke.
Without further information, it's difficult to say for sure what "Nat Su Cow" means. Could you provide more context or details about where you came across this phrase?