It is possible for both hosts to send fins simultaneously then both just have to ack.
Word Origin and History for ack
British oral code for letter A in wireless and telephone communication, 1898; hence ack-ack "anti-aircraft" (gun, fire, etc.).
ack in Technology
1. /ak/ The mnemonic for the ACKnowledge character, ASCII code 6. 2. A message transmitted to indicate that some data has been received correctly. Typically, if the sender does not receive the ACK message after some predetermined time, or receives a NAK, the original data will be sent again. [Jargon File] (1997-01-07)