lag vs get behind

lag is a noun but get behind is not a noun.

Word	Noun	Adjective	Verb	Adverb
lag	Yes	No	Yes	No
get behind	No	No	Yes	No

As verbs, get behind is a hyponym of lag; that is, get behind is a word with a more specific, narrower meaning than lag:

Other hyponyms of lag include drag, drop back, drop behind, hang back, trail.

lag (noun)	get behind (noun)
the act of slowing down or falling behind
one of several thin slats of wood forming the sides of a barrel or bucket
the time between one event, process, or period and another

lag (verb)	get behind (verb)
throw or pitch at a mark, as with coins	to lag or linger behind
cover with lagging to prevent heat loss
hang (back) or fall (behind) in movement, progress, development, etc.
lock up or confine, in or as in a jail