The Hebrew word tam means perfect, in the sense of being whole, complete, blameless and mature.

This word is used to describe Job:

KJV Job 1:1 There was a man in the land of Uz, whose name was Job; and that man was perfect and upright, and one that feared God, and eschewed evil.

In the King Kames Bible the word tam is translated as perfect nine times, undefiled twice and upright once, and each of these translations are in-line with the meaning of the Hebrew word tam.

but, one time this word is translated very differently.

KJV Genesis 25:27 And the boys grew: and Esau was a cunning hunter, a man of the field; and Jacob was a plain man, dwelling in tents.

In this passage the King James translation diverges from the meaning of tam and instead uses a word that has nothing to do with the meaning of the Hebrew word tam. Why weren't the King James consistent and translated the above as , 'Jacob was a perfect man,' like they did for Job in Job 1:1?

I found out a long time ago, that when the King James Translates a verse that is not consistent with the Hebrew, all you have to do is look to the Septuagint to find out why they translated a particular verse the way they did.

When we look at Job 1:1 again, we find that the Septuagint translated the Hebrew word tam with the Greek word αμεμπτος, which means perfect, the same as the Hebrew word tam and the King James Translation is consistent with the Hebrew and the Greek.

But when we examine Genesis 25:27 we find that the Greek translated the Hebrew word tam as απλαστος, which means plain or simple and the King James Translation bypasses the Hebrew and instead follows the Greek Septuagint in its translation.