megrim vs sick headache

megrim and sick headache both are nouns.

Word	Noun	Adjective	Verb	Adverb
megrim	Yes	No	No	No
sick headache	Yes	No	No	No

As nouns, megrim and sick headache are synonyms defined as:

megrim and sick headache: a severe recurring vascular headache; occurs more frequently in women than men

Other synonyms of megrim include hemicrania, migraine.

megrim (noun)	sick headache (noun)
a severe recurring vascular headache; occurs more frequently in women than men	a severe recurring vascular headache; occurs more frequently in women than men
	a headache accompanied by nausea