put behind bars vs lag

put behind bars is not a noun while lag is a noun.

Word	Noun	Adjective	Verb	Adverb
put behind bars	No	No	Yes	No
lag	Yes	No	Yes	No

As verbs, put behind bars and lag are synonyms defined as:

Other synonyms of put behind bars include gaol, immure, imprison, incarcerate, jail, jug, put away, remand.

put behind bars (noun)	lag (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

put behind bars (verb)	lag (verb)
lock up or confine, in or as in a jail	throw or pitch at a mark, as with coins
	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