Durin's Folk are also known as the "Longbeards", which translates to Sigin-tarâg in Khuzdul.
Although we aren't given a gloss of the word, it makes sense that tarâg is "beards", seeing as how CaCâC is a well know plural pattern. We can assume that it is plural, nominative, and indefinite as well.
Sigin is then "long", and probably singular, nominative, composition. It may actually be a verbal root (a "stative verb") similar to gabil. However, it's hard to explain the different vocalization compared to gabil. The adjectival form of stative verbs in Arabic are mostly of the pattern CaCîC, but some have the form CâCiC found in active participles. It may be significant that the basic form of dynamic verbs is almost always CaCaCa. Khuzdul has the dynamic verbs felek and gunud, and felak can also be used as a verb. That shows that Khuzdul verbs forms are not as regular as seen in Arabic, which probably includes stative verbs as well. This shows that sigin may simply be the verbal adjective of a different verbal stem pattern than gabil and gamil.
Sigin-tarâg is "Longbeards" and a compound word with adjective-noun word order.
The Peoples of Middle-earth, pg 321