sit down vs crouch

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

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

As verbs, crouch is a hyponym of sit down; that is, crouch 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
crouch: sit on one's heels

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

sit down (noun)	crouch (noun)
	the act of bending low with the limbs close to the body

sit down (verb)	crouch (verb)
show to a seat; assign a seat for	sit on one's heels
be in a position in which one's upper body is largely upright and supported by one's backside	bend one's back forward from the waist on down
take a seat