flag vs flag down

flag is a noun but flag down is not a noun.

Word	Noun	Adjective	Verb	Adverb
flag	Yes	No	Yes	No
flag down	No	No	Yes	No

flag (noun)	flag down (noun)
a conspicuously marked or shaped tail
emblem usually consisting of a rectangular piece of cloth of distinctive design
stratified stone that splits into pieces suitable as paving stones
flagpole used to mark the position of the hole on a golf green
a rectangular piece of fabric used as a signalling device
a listing printed in all issues of a newspaper or magazine (usually on the editorial page) that gives the name of the publication and the names of the editorial staff, etc.
plants with sword-shaped leaves and erect stalks bearing bright-colored flowers composed of three petals and three drooping sepals

flag (verb)	flag down (verb)
provide with a flag	signal to stop
become less intense
communicate or signal with a flag
decorate with flags
droop, sink, or settle from or as if from pressure or loss of tautness