IPA: /ˈsɪkli/
KK: /ˈsɪkli/
In a way that is weak, unhealthy, or affected by illness.
She smiled sicklily, trying to hide her discomfort.
Sicklily → It is formed from "sick" (from Old English "sic", meaning ill or unwell) and "-ly" (a suffix meaning in a manner of). The word "sicklily" means in a manner that is sickly or ill.
Think of feeling 'ill' ('sick') and doing so 'in a manner' ('-ly') — that's why sicklily means behaving in a sickly way.