Lexical Definition

‎רַעְמָה‎ raʻmâh (rah-maw') mane ^H:N-F
feminine of ‎רַעַם‎

thunder.

• the mane of a horse (as quivering in the wind)

1) vibration, quivering, waving, mane (of horse)
1a) meaning uncertain

Brown-Driver-Briggs

I. ‎רַעְמָה‎ (Gi^{Intr. 127 f.}) noun feminine vibration? quivering mane? of horse's neck: Job 39:19 hast thou clothed his neck (with) ‎׳‎‎ר‎? so most, but very uncertain.

Brown-Driver-Briggs References