The story
ARNK: short for Arrival Not Known (a.k.a. arrival unknown): It is a four-letter code sometimes shown as a flight segment on a reservation with multi-flight segments: where the traveler(s) depart from an airport, which they did not fly into with that specific airline (e.g. traveled there by car or used different airline etc.) - For example: if the flights are from IAH>FLL & MIA>IAH (return): -- The ARNK indicator (flight segment placeholder/indicator) is used by the airline to indicate, as confirmation, that the customer has other transport arrangements between the 2 flight segments that are not known to the airline (In essence a confirmation that the flight(s) were not changed/updated incorrectly or deleted by accident by for example user/customer/system etc. but intentional booking by customer). --- For example: IAH>FLL, ARNK, MIA>IAH
Etymology from Wiktionary, CC BY-SA 4.0.