Like the other people are saying, it means" flat" or "broken outer surface".

Are you familiar with poker? We have another use of the word `flush` in that context, https://en.wikipedia.org/wiki/List_of_poker_hands#Flush where all of the cards are the same suit. So it's a similar use of flush in this instance, that everything is the same. It's similar to this thread's use of "flush".