The form lhegin in the published Etymologies might be a misreading for lhegrin, see VT/45:25. As noted by Bertrand Bellet, the two forms are listed side by side, and they may simply be doublets, but it is also possible that we have here a singular followed by its plural.
Noldorin
celeg
adjective. swift, agile, hasty
lhagr
adjective. swift, rapid
The form lhegin in the published Etymologies might be a misreading for lhegrin, see VT/45:25. As noted by Bertrand Bellet, the two forms are listed side by side, and they may simply be doublets, but it is also possible that we have here a singular followed by its plural.