Anathema sit, a Latin phrase (though the word anathema is Greek, it is used within the context of Latin grammar, hence the expression is in Latin). I could simply translate it as let him/her be anathema, or even let him/her be excommunicated, but that would not explain the very specific usage of this phrase over the last two millennia.
This phrase has been used by the highest authorities of the Catholic Church, such as ecumenical councils and Popes to declare a teaching that is to be considered infallible. Contrary to common opinion, most of the teachings of ecumenical councils and Popes have not been declared as infallible.
The expression is used when some doctrine is to be infallible--and that is actually a fairly modern interpretation, for centuries it was simply a doctrine of utmost importance that no one would dare question, but the idea of infallibility only dates back to the First Vatican Council which, in 1870, infallibly declared the Pope infallible in the matters of faith and morals when, and only when, he is formally declaring a doctrine, i.e., when he says "I am now declaring this infallibly." He would usually not say it explicitly, but would use an anathema sit construct.
Specifically, these declarations are usually made negative, that is to say, they do not declare a truth but condemn an untruth. That idea is that the declaration affirms something that the Church has always believed and condemns a heretical challenge to such truth. In reality the Catholic Church has evolved over time (and it does not try to deny that), so new dogmas do not necessarily reflect an ancient tradition, but the idea definitely is to tie any new dogma into the Catholic tradition.
The wording of such a declaration usually starts with the words "if one would to say that" and end with "anathema sit". The anathema sit is often left untranslated even if the rest of the document is translated.
To show an example, I did a web search on anathema sit, and will quote the very first occurrence that I found. It happened to be an appendix to the minutes of the First Council of Toledo, found at http://freespace.virgin.net/angus.graham/BragaApp.htm, and it says:
Si quis autem dixerit aut crediderit a Deo omnipotente mundum hunc factum non fuisse, atque eius omnia instrumenta, anathema sit.
This can be translated into English as If anyone would then say or believe that this world, and everything on it, was not created by the omnipotent God, anathema sit.