Ancient meaning of the the character 思 The Analect 2.2 quotes from the Book of Odes, and Arthur Waley claims there is a difference between how Confucius there reads the character 思 (sī, think) and how it was used in the Ode.  Waley claims that in the Ode &quot;ssu does not mean 'thoughts', but is an exclamation, 'oh', 'ah', or the like.&quot;  I looked at the various old dictionary definitions, and textual examples on the Chinese Text Project https://ctext.org/ and I did not find any evidence there that 思 was ever used the way Waley says. It always seems to mean &quot;thought(s)&quot; or &quot;think.&quot;  Other searches got me nothing relevant.
Is there evidence supporting Waley's claim?
Arthur Waley is indeed reflecting Chinese scholarship with this statement. In the Book of Odes 《詩經》, when 「思」 appears at the end of a line, it may be interpreted as a  modal particle (語气詞), equivalent to modern Mandarin 「啊」. For example,

《國風・漢廣》
漢之廣矣、不可泳思。
江之永矣、不可方（汸、舫）*思。
How broad, the Waters of the Hàn - they cannot be swum! 
How long, the Waters of the Jiāng - they cannot be rafted across!*
*《說文・方部》：「方，併船也。象兩舟省緫頭形。汸，方或从水。」

《小雅・鹿鳴之什・采薇》
昔我往矣、楊柳依依。
今我來思、雨雪霏霏。
Long ago, when I departed, the willows were fresh and tender;
Now, as I've returned, the rain and snow fall in torrents.


Please note that this use of 「思」 is not its earliest or original meaning. Apart from being a proper noun for a geographical region or person, 「思」's earliest usage is the same as its modern usage, meaning thoughts, contemplation, deliberation. From Guōdiàn Chǔ Slips: The Five Conducts, Slip #9:


《郭店楚墓竹簡・五行》09
……（不）（仁），恖（思）能清（精）。（智），恖能倀（長）……
...Without benevolence, one's thoughts cannot be refined; without wisdom, one's thoughts do not have foresight...


References:

《漢語大詞典》
黃德寬《古文字譜系疏證》
何琳儀《戰國古文字典：戰國文字聲系》