"infected with measles," 1680s, from measle (see measles) + -y (2). The Middle English word for "infected with measles" was maseled. Sense of "meager and contemptible, good for nothing" is attested by 1864 in British slang.
infectious disease causing eruptions of rose-colored papulae, early 14c., plural of Middle English masel "little spot,"which isperhaps from Middle Dutch masel "blemish" (in plural "measles") or Middle Low German masele, both from Proto-Germanic *mas- "spot, blemish" (source also of Old High German masla "blood-blister," German Masern "measles").
There might have been an Old English cognate, but if so it has not been recorded. "The phonetic development is irregular" [OED] and the form might have been influenced by Middle English mēsel "leprous; a leper; leprosy" (late 13c., obsolete from mid-16c.), which is from Old French mesel and directly from Medieval Latin misellus "a wretch," noun use of an adjective meaning "wretched," a diminutive of Latin miser "unhappy, wretched, pitiable, in distress."
adjective suffix, "full of or characterized by," from Old English -ig, from Proto-Germanic *-iga- (source also of Dutch, Danish, German -ig, Gothic -egs), from PIE -(i)ko-, adjectival suffix, cognate with elements in Greek -ikos, Latin -icus (see -ic). Originally added to nouns in Old English; used from 13c. with verbs, and by 15c. even with other adjectives (for example crispy).
Others are reading
<a href="https://www.etymonline.com/word/measly">Etymology of measly by etymonline</a>
Harper, D. (n.d.). Etymology of measly. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/measly