Abarim Publications' online Biblical Hebrew Dictionary
כלח
The assumed root כלח (klh) does not occur as verb in the Bible but in Arabic a comparable verb appears meaning to look hard or stern, or rather: to contract the face. The Hebrew root appears in the Bible through the masculine noun כלח (kelah), probably denoting vigor or hardness (rugged of strength). It occurs only in Job 5:26 and 30:2.