What does in mean in the following sentence?
MTV Europe Music Awards nominations are in and Lady Gaga leads
It feels like "announced", but does it refer to "in the news" or something? Where does it come from?
This sense of in means
5 (of letters, etc.) received
Applications must be in by April 30.
Your example, nominations are in and Lady Gaga leads, makes it clear that the nominations have not merely been sent in, but already received by the judges, read, and tallied. However, sometimes a phrase like this refers more to the time of submission, implying that the item will also be received at that time, but might not be handled until later. For example:
The entries are in and we'll have the results next week. [The entries were received but not judged.]
Get your votes in by noon on the 30th! [Submit your votes by this time.]
This use of in often relates to judging—note the words like nominations and votes—but you might use it when referring to anything that is sent or submitted.
I already sent in the information you wanted. [I submitted it, to someone who will receive it.]
Let me check—sorry, we didn't get that in yet. [We haven't received it.]
The in in the sentence refers to the nominations having been submitted and appropriately received. We know they were received because there is an official tally in which Lady Gaga leads.
Etymologically, using the preposition in likely refers to a long-standing tradition among organizations to convene nominating people and things for awards, elections, and whatever by having those empowered to nominate submit their nominations by putting them in a hopper. When nominations are in the hopper, they are in. Colloquially and idiomatically, the preposition in is still used even if there is no hopper.