what is the mekor and reason not to cut nails on Wednesday?

Not Thursday?

So THAT explains what happened earlier.
I was cutting my shiksa's nails, and I apparently cut too much and she started bleeding

He must mean Wednesday night when it's already Yom Chamishi.

Joshman, look at the Mishnah Berurah OC 260 s'k 6 and at the Shulchan Aruch Harav OC 260:2.

Is this some kind of takanah of R'Yehuda Hachassid?

I looked in the Sefer Chassidim and didn't see it mentioned there. I think the practice post-dates the Sefer Chassidim by at least a couple hundred years. The Ba'ch to OC 260 quotes R. Shlomo Luria as the source.
The reason is that the nails start to grow on the third day after cutting them, so if you cut them on thursday, they will start to grow back on shabbat, and for some reason that's not ideal.




As always, ask one's own local orthodox rabbi and don't rely on the rantings that I derive from general ignorance and stalk-trolling internet fora.

How is it different from spreading seeds that will start growing on the third day?

I have no idea. Are you not supposed to spread seeds on Thursdays?

On the contrary. AFAIK it's entirely permitted and one should not worry that it will start to grow on Shabbos.

What is the source the seeds will start growing the third day?

יב וַתּוֹצֵא הָאָרֶץ דֶּשֶׁא עֵשֶׂב מַזְרִיעַ זֶרַע לְמִינֵהוּ וְעֵץ עֹשֶׂה-פְּרִי אֲשֶׁר זַרְעוֹ-בוֹ לְמִינֵהוּ וַיַּרְא אֱלֹהִים כִּי-טוֹב:
יג וַיְהִי-עֶרֶב וַיְהִי-בֹקֶר יוֹם שְׁלִישִׁי:

(I of course meant on the third day of it being planted...)

Not all seeds. However, if one has a certain type of seed like that, then [etc].

Isn't there something about the plants (and everything else) having been formed in their potential on the first day which means they actually sprouted on the third day of their creation?

Does she still talk to you?

You mean like Rashi on Breishis 1:14 ?