The Hebrew word תָּם (tām) occurs 13 times in the Hebrew Bible, and appears to suggest peaceableness and originality related to direct and personal relationship to the LORD, since each of the 13 citations in the Bible refers to individuals in this regard.
The following citations reinforce this outlook.
Philo of Alexandria (1st Cent)
Ἰακὼβ δὲ ἄπλαστος ἄνθρωπος οἰκῶν οἰκίαν (Leg. Allegor. iii. § 1, i. 87, 88).
Ἰακὼβ γὰρ ἄπλαστος οἰκῶν οἰκίαν εἰσάγεται (De Plantat. Noe, § 11, i. 336).
Μαρτυρεῖ δὲ Μωϋσῆς φάσκων, ὅτι ἦν Ἰακὼβ ἄπλαστος, οἰκῶν οἰκίαν (De Congr. Erud. grat. § 12, i. 528).
In three places, Philo uses the same Greek word used for the Greek translation of the Hebrew Bible (LXX), ἄπλαστος, which literal meaning is “not capable of being moulded” (i.e., natural, unaffected, ingenuous).
Targum Onqelos to the Pentateuch (2nd/3rd Cent)
וּרבִיוּ עוּלֵימַיָא וַהֲוָה עֵשָׂו גְבַר נְחַשִירכָן גְבַר נָפֵיק חֲקַל וְיַעֲקֹב גְבַר שְלִים מְשַמֵיש בֵית אוּלפָנָא׃
Targum Neofiti to the Pentateuch (4th Cent?)
ורבו טליא והוה עשו גבר ידע צידה גבר מרי חקלין ויעקב הוה גבר שלם בעבדא טבא יתיב בבתי מדרשא׃
Both Targumim use the Aramaic phrase גְבַר של[י]ם, which means “man of peace.”
Of interest, the marginalia of this latter Targum contains the following note:
גבר ידע... (גיבר) גבר רמאי הוה ומן ציידיה הוה טעים ליה בפימיה ורבקה רחימת ית יעקב מן בגלל (דעינוותן) הוא ומשמש בבית מדרשא אולפן דאוריית׳
Man of skill: (The strong) man [Esau] was a fraud, and from wild game was there
flavor in his mouth, but Rachel loved Jacob because (that modest person) he was,
and he applied learning of Torah in the house of study.
The rabbis here used the Aramaic word עינוותן, which means “modest person.”
Jerusalem Talmud (4th/5th Cent)
ואין איש אלא יעקב כמה דתימר ויעקב איש תם...
The Aramaic word here is תָּם (tām), which would be the equivalent of the same Hebrew word noted above.
Rabbi Shlomo Yitzchaki (Rashi) (12th Cent)
Rashi makes the following commentary on the word תָּם (tām):
תָּם: He was not an expert in all these [matters]. Like his heart, so was his mouth. A person who is not astute at deceiving is called תָּם, innocent.
In summary, these early translations by Jews into Greek and Aramaic in addition to other commentary by those familiar with Jewish oral tradition appear to suggest that the adjective here for Jacob would describe the peaceableness and originality of character that come from direct and personal relationship to the LORD. This originality of character is why the first endearing romantic encounter ever mentioned in the Bible appears between Jacob and Rachel.
However, the peaceable character of Jacob did not prevent him from “tripping up” others in order to gain competitive advantages related to obtaining blessings as the meaning of his name would imply. That is, his name was changed to Israel because he finally was striving with God (יִשְׂרָאֵל) in order to obtain those blessings made to Abraham. That is, after the physical engagement of Jacob with the (Angel of the) LORD, the LORD started to refer to himself in Scripture as the God of Abraham, Isaac, and Jacob.