Rukhs / Rakhâs

Quite simply, these are the Khuzdul words for "Orc", singular, and "Orcs", plural, respectively.  It was the first example of a Khuzdul singular and plural form released to the public, found in The War of the Jewels.

Rukhs is "orc" and singular, nominative, indefinite.

Rakhâs is "orcs" and plural, nominative, indefinite.

The War of the Jewels, pg 391