From an unused root meaning to be complete; maturity -- full (old) age.
H3624. kelach
I. כֶּ֫לַח noun masculine possibly firm or rugged strength (see √ above), בְכ ׳תָּבוֺא אֱלֵיקָֿ֑בֶר Job 5:26 thou shalt come in firm strength (with thy body vigorous, powers unimpaired) unto the grave; כָּ֑לַךְ אָבַד עָלֵימוֺ 30:2 upon whom vigour has perished.