The fingernail of the earlier generations is better than the belly of the later generations/ our forefathers were much wiser than we ("their finger-nail was worth more than the belly of later generations")
By way of analogy: the earlier generations are far greater in stature than the later generations.
("For those of the earlier generations whose sin was revealed, their end was revealed; for those of the later generations whose sin was not revealed, their end was not revealed." Rabbi Yohanan said: "Better the fingernail of the earlier ones than the belly of the later ones." Reish Lakish said to him: "On the contrary, the later ones are preferable." - Yoma 9b)
Translation
Expression (Hebrew)
טוֹבָה צִפָּרְנָם שֶׁל רִאשׁוֹנִים מְכַרְסֵם שֶׁל אַחֲרוֹנִים
Definition (Hebrew)
בְּהַשְׁאָלָה: הַדּוֹרוֹת הַקּוֹדְמִים עוֹלִים בְּהַרְבֵּה בִּגְדֻולָּתָם עַל הַדּוֹרוֹת הָאַחֲרוֹנִים.
("רִאשׁוֹנִים שֶׁנִּתְגַּלָּה עוֹנָם - נִתְגַּלָּה קִצָּם, אַחֲרוֹנִים שֶׁלֹּא נִתְגַּלָּה עֲוֹנָם- לֹא נִתְגַּלָּה קִצָּם. אָמַר רַבִּי יוֹחָנָן: טוֹבָה צִפָּרְנָן שֶׁל רִאשׁוֹנִים מִכְּרֵסוֹ שֶׁל אַחֲרוֹנִים. - אֲמַר לֵהּ רֵישׁ לָקִישׁ: אַדְּרַבָּה, אַחֲרוֹנִים עֲדִיפֵי"- יוֹמָא ט ע"ב)