- What is the source for referring to a rabbinic lecture as a "shiur" which literally means 'measurement'?
- When did this usage begin?
- What is its significance?
The meaning "measure" came first, and only in medieval Hebrew did shiur come to mean lesson - "a set measure of learning" (that sense is preserved in shiurei bayit שיעורי בית - "homework"), followed by the modern Hebrew sense of "class". The word shiur derives from the root שער meaning, "to calculate, to estimate, to measure". The verb form only appears once in Tanach, in Mishlei 23:7. The noun form also only appears once, but for me in a surprising location, Bereshit 26:12
וַיִּזְרַע יִצְחָק בָּאָרֶץ הַהִוא, וַיִּמְצָא בַּשָּׁנָה הַהִוא מֵאָה שְׁעָרִים Yitzchak sowed in that land and in that same year found meah she'arim
Source with more details: http://www.balashon.com/search?q=shiur
Orginally suggested by Gerhon Gold in the comments
See the Sefer Yisroel Veorisa who writes that it refers to the measurements people set for themselves. The Achronim write to set a measurement in chummash, mishnah etc.
It seems that people had set times to hear a daily or weekly shuir so it got that name.