No. I've never heard it in English either. At least not in British English. Instead we say "he/she's having kittens", when somebody is really stressed/anxious about a situation.
But you can say (in German) Du blöde Kuh! (You stupid cow!) That's the only "cow" expression that comes to mind... there must be more... I shall ruminate on it...
Must be an American English idiom then, though I can say it is not commonly used anymore. Now people just say "chill" or "don't trip" as in "calm down." The "kittens" expression is interesting, I've never heard that one.
Thanks for the German bit; learn something new everyday! :-)
Use eine before a feminine word such as Kuh "cow" or Katze "cat" or Person "person" or Gabel "fork".
Use ein before a neuter word such as Pferd "horse" or Mädchen "girl" or Messer "knife".
Also use ein before a masculine word such as Hund "dog" or Apfel "apple" or Löffel "spoon", when it is the subject of a verb - the thing that is or does something.
Use einen before a masculine word that is the object of a verb - the thing that gets acted on.
Whether a word is masculine or feminine or neuter is something that you just have to learn -- ideally, when you learn the word itself, you learn it with its definite article, e.g. not just Apfel = apple but der Apfel = the apple so that you will know that it is masculine.
could you say 'sie hat ein kuh gehabt' and it mean the same thing?
It would still have to be eine Kuh with -e on eine and with a capital K on Kuh.
Otherwise yes, sie hat eine Kuh gehabt is possible -- but haben is one of those verbs that is still commonly used in the simple past, so you will probably find sie hatte eine Kuh more often.
Lower-case "sie" only means "you" (in reference to a female) in very old usage.
Upper-case "Sie" is a polite way to say "you" (comparable to "Lei" in Italian), either in singular or plural. It uses third-person plural conjugation, though -- so "Sie haben eine Kuh" could be either "They have a cow" or "You (polite/formal; one or many) have a cow".
- Heute essen sie Brot = Today they eat bread
- Heute essen Sie Brot = Today you (formal) eat bread
- Sie essen heute Brot = They eat bread today -OR- You (formal) eat bread today
If it's at the beginning of a sentence, you can't tell the difference anymore because the first letter of a sentence is always capitalised, as in English. So the 'they' word 'sie' and the formal 'you', 'Sie' will both be 'Sie' at the beginning.
Wiktionary shows Rind as a generic term for a bovine (includes cows, bulls, and steers), whilst Kuh is specifically a female bovine (a cow). It also shows Rind as a term for "beef", the edible form of a bovine.
dict.cc shows similarly, but indicates that the sense of "beef" is more common than the use a term for cattle.
The article needs to be declined to match the noun it is being used with in number, case, and gender. In the construction "[Jemand] hat [etwas]", etwas is Akkusativ.
Katze are both feminine;
Hund is masculine.
If you've reached level 14 without picking this up, die Eule has been too generous in awarding progress. You could remedy this by looking at these two discussions (and possibly others):