Sickly

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late Middle English: probably suggested by Old Norse sjúkligr .

wiktionary

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From Middle English seekly, sekely, siklich, sekeliche, equivalent to sick +‎ -ly. Possibly a modification of Old English sīcle(“sickly”) and/or derived from Old Norse sjúkligr(“sickly”). Cognate with Dutch ziekelijk, Middle High German siechlich, Danish sygelig, Swedish sjuklig, Icelandic sjúklegur.

etymonline

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sickly (adj.)

late 14c., "ill, invalid, habitually ailing," from sick (adj.) + -ly (1). Meaning "causing sickness" in any sense is from c. 1600. Related: Sickliness.