sit down vs lounge

sit down is not a noun while lounge is a noun.

Word	Noun	Adjective	Verb	Adverb
sit down	No	No	Yes	No
lounge	Yes	No	Yes	No

As verbs, lounge is a hyponym of sit down; that is, lounge is a word with a more specific, narrower meaning than sit down:

sit down: be in a position in which one's upper body is largely upright and supported by one's backside
lounge: sit or recline comfortably

Other hyponyms of sit down include sprawl, perch, rest, roost, crouch, hunker, hunker down, scrunch, scrunch up, squat.

sit down (noun)	lounge (noun)
	a room (as in a hotel or airport) with seating where people can wait
	an upholstered seat for more than one person

sit down (verb)	lounge (verb)
show to a seat; assign a seat for	sit or recline comfortably
be in a position in which one's upper body is largely upright and supported by one's backside	be about a place without any apparent purpose
take a seat