Who is פלוני in reference to, in the following quote from Tosafot:
הוה שכיח גביה מלאך המות. דמספר מה שאירע לו כבר דהאי עובדא דמרים מגדלא נשייא בבית שני היה דהיתה אמו של פלוני כדאיתא בשבת (דף קד: ושם):
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The Ploni referred to here is ben Stada who appears in the gemara in Shabbat 104b and in Sanhedrin 67a, described there, among other things, as the son of Miriam, grower of women's hair (megadla se'ar nashia). Whether this is a talmudic reference to Jesus is a matter of some dispute (see here, here and here, for example), even among the Tosafists themselves. Rabbeinu Tam, for one, thought ben Stada was Papus ben Yehuda, a troublemaker from the time of Rabbi Akiva (several decades after the NT's Jesus; certainly several decades after the time of the Second Temple, which is the view held by the Tosfot here in Chagigah).
However, as the ben Stada sections were censored by the Church because they held the position that these sections do, in fact, refer to Jesus and are attempting to mock him, it makes sense that this Tosfot would likewise be censored. Perhaps in the original version it said "mother of ben Stada", or perhaps the Tosfot themselves wrote this way, to bypass the censor.