English term (edited): put one's stamp on

While willmatter's explanation also exists, unfortunately, it's not the right one for your context in this case.

Lots of useful info on your subject in http://en.wikipedia.org/wiki/Ger_(Hasidic_dynasty)

After the death of the Kotsker Rebbe in 1859 the vast majority of his chasidim chose Rabbi Yitschok Myer Alter, the Kotsker Rebbe's brother-in-law and his closest disciple, as their new rebbe (leader). At the time Rabbi Alter lived in Warsaw and led the main Kotsker shtibl there (on ul. Zelazna). Shortly after acceding to the leadership of the chasidim he was appointed as the Town Rabbi (ABD) of Ger. He moved to Ger, thus becoming the **founding rebbe of the Gerrer dynasty**. During his seven years of leadership, the chasidus flourished, causing it to be known as the 'seven years of plenty'[1].

He was the 1st Gerrer Rebbe.
Grand Rabbi Yitzchak Meir Alter (1798 - March 10, 1866), also known as the Chidushei HaRim. Notable student of the Kotzker Rebbe and a prominent contemporary decisor of Jewish Law. Assumed leadership of the chasidim in 1859.

See also http://en.wikipedia.org/wiki/Yitzchak_Meir_Alter

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So anyway, back to the meaning of "put one's stamp on" in this context...
This 1st Gerrer Rebbe was extremely influential. He "shaped" the chasidim （= 'putting his stamp on it'). In other words, he heavily influenced the chasidim through his strong leadership skills and charismatic personality. He "stamped" many of his own characteristics, much of his own personality, on the chasidim.

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Note added at 9 hrs (2007-10-24 01:26:02 GMT) Post-grading
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度々すみません。
"Put one's stamp on" is similar to another idiom: "to leave one's mark on". Lots of examples of this in Eijiro:
* leave one's mark on
～に強い影響｛えいきょう｝を与える
・The abnormal weather left its mark on the people in that area. : 異常気象は、その地域の人々に大きな影響を与えた。
* leave one's mark on history
歴史にその名を残す
* leave one's mark on public policy
公共政策｛こうきょうせいさく｝に影響｛えいきょう｝を残す［与える］
・The devastating fire left its mark on public policy. : その壊滅的な大火事は公共政策に影響を残した。
* leave one's mark on the world
この世に自分｛じぶん｝の足跡｛あしあと｝を残す
* leave one's mark
影響を与える、足跡を残す


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Note added at 11 hrs (2007-10-24 03:20:41 GMT) Post-grading
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Hi Seika. Yes, that's a good way to describe him!