An adjective appearing as N. flaew “sickly, sick, ill” in The Etymologies of the 1930s derived from ON. thlaiwa < ᴹ✶slaiwā under the root ᴹ√SLIW “sickly” (Ety/SLIW). The ancient initial sl became thl which was the usual sound change in (Old) Noldorin, and then this thl became fl, a less common Noldorin sound change. Tolkien wrote an intermediate form thlaew between ON. thlaiwa and N. flaew, but then revised this form to thloew, reflecting some uncertainty on the phonetic development of ai in Noldorin. There was also a deleted variant of this root: ᴹ√LIW, where Tolkien had lhaew “ill” with the usual unvoicing of initial l in Noldorin (EtyAC/LIW).
Neo-Sindarin: In Sindarin of the 1950s and 60s, initial sl became lh instead of thl, so most Neo-Sindarin writers adapt this word as ᴺS. lhaew “sickly, sick, ill”, as suggested by Hiswelókë’s Sindarin Dictionary (HSD).