The original meaning for the word תם - wholesome, as explained by Rashi on the verse Devarim 18:13:
You must be wholehearted/perfect (תמים) with your God יהוה, i.e. walk before him whole-heartedly, put thy hope in Him and do not attempt to investigate the future, but whatever it may be that comes upon thee accept it whole-heartedly, and then thou shalt be with Him and become His portion (Sifrei Devarim 173:3).
There are also explanations based on reading the word תם backwards, i.e. מת - dead. I.e. תם is the opposite in every respect of being dead.
In life mostly simple people tend to be wholehearted (hence the modern usage and usage in Passover Hagadah). However, Yaakov avinu was very a smart and learned person for his time, and he remained wholesome despite his trials in life.
The etimilogical dictionary by Ernest Klein translates תם as
1 complete, perfect. 2 innocent, artless. PBH 3 naive. PBH 4 without blemish. [From תמם.] Derivative: תַּמּוּת.
where PBH = Post Biblical Hebrew. Perhaps, the post biblical meaning has been acquired due to Passover Haggadah, where the 3rd son is asking a question, which appears simpler compared to those of the first two sons. Talmud Yerushalmi Pesachim 10:4 names the sons as
תַּנֵּי רִבִּי חִייָה. כְּנֶגֶד אַרְבָּעָה בָּנִים דִּיבְּרָה תוֹרָה. בֶּן חָכָם בֶּן רָשָׁע בֶּן טִיפֵּשׁ בֶּן שֶׁאֵינוֹ יוֹדֵעַ לִשְׁאָל.
Rebbi Ḥiyya stated “The Torah spoke about Four Children, a wise child, a wicked child, a stupid child, and a child who does not know how to ask.
where, the word טִיפֵּשׁ means stupid. The same wording is used in Mekhilta d'Rabbi Yishmael 13:14:1 quoted by Rashi in the commentary on Chumash where he explains the verse quoted by the child in the Haggada Shemos 13:14:
מה זאת WHAT IS THIS? — This is the question of a dull child (תינוק טפש) who has not sufficient understanding to question very profoundly and who therefore asks in an indefinite fashion, “What is this?” In another passage (Deuteronomy 6:20) it states, “[When thy son asketh thee…], What mean the testimonies and the statutes and the judgments … [which the Lord our God hath commanded you?]” This, however, is the question of a wise son. The Torah in mentioning four different explanations of the Passover sacrifice-rite to be given by a father to his children, is speaking in reference to four different types of son: the wicked son (Shemos 12:25 and in the second half ofShemos 13:8), and one who has not sufficient understanding how to ask (טִיפֵּשׁ, in the first half of Shemos 13:8), and one who asks in an indefinite manner (Shemos 13:14), and one who asks in a wise fashion (Devarim 6:20) (cf. Mekhilta d'Rabbi Yishmael 13:14:1; Jerusalem Talmud Pesachim 10:4).
The Passover Haggadah changed the wording, however, the old understanding continued:
כְּנֶגֶד אַרְבָּעָה בָנִים דִּבְּרָה תוֹרָה: אֶחָד חָכָם, וְאֶחָד רָשָׁע, וְאֶחָד תָּם, וְאֶחָד שֶׁאֵינוֹ יוֹדֵעַ לִשְׁאוֹל.
The Torah relates to four types of sons – one who is wise, one who is wicked, one tam - with a simple nature, and one who does not know how to ask.
Vilna Gaon in his commentary on Haggadah mentions that this is a simple understanding. The message of the Haggada is deeper. I.e. the sons are not grouped linearly by their ability to ask a question, but in pairs:
חכם - a wise son vs. שֶׁאֵינוֹ יוֹדֵעַ לִשְׁאוֹל - one who cannot ask;
רָשָׁע - a wicked son vs. תָּם - a wholesome son
Indeed, a wholesome son is not going to strike others with the most complex question: he is interested in the whole experience of the seder, just like the wicked son seeks to separate himself from the seder. Perhaps, that was the intent to change the word טִיפֵּשׁ to תָּם.
