Definition of cold
Literally, "cold". The term "cold" is often used at the poker tables in a variety of contexts. There's "cold deck", which means you haven't had a decent hand in a long time. And there are the expressions "cold call", "cold 3bet", "cold 4bet", etc. What is generally meant by "cold" is that one does not close the action. For example, a conservative player who has not played a hand in a long time and opens in UTG position. A player on the cutoff 3 bets the opening and you, on the small blind, over-raise (4bet). In this situation, it is called a "cold 4bet".
Implications of cold 4bet
There are no absolutes in poker, but let's just say that generally, the phrase "cold 4bet" in our example above indicates a lot of strength. If we go back to our example of the small blind, we go with a cold 4bet, meaning we don't close the action. The UTG player may have a very big hand, the 3bet player may have a very big hand. The player who "cold 4bet" does not know what the other players are going to do and has no clue that a player is weak. In short, "cold" is used when a player does not close the action. If our hero above had called instead of 4bet, we would have said he "cold called the 3bet".