an armed plainclothes federal marshal riding on an airliner to protect against skyjacking.
Marshal vs. MartialMartial can only be used as an adjective, but marshal can be used as either a noun or a verb, but not an adjective.
Origin of sky marshal
An Americanism dating back to 1965–70
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for sky marshal
an armed security guard on a commercial aircraft
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012