Superlative form of sickly: most sickly.(adjective)
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Use "sickliest" in a sentence
"Blinded in his right eye just after birth when a midwife rinsed his eyes with a contaminated fluid, Mr. Jordan was the smallest and sickliest of 15 children born to migrant farm worker parents."
"The sickliest states are among the first to be acting, and Illinois certainly qualifies."
""Of all British institutions it now appears one of the sickliest," David Hare wrote in 1994, in a Charter 88 paperback called The Power and the Throne."