Is the shiur the size of an egg properly called a k'beitzah ("כביצה") or a "kebeyah" ("כביעה" maybe)? If "כביצה" is correct, as I think it is, then why do people sometimes say "כביעה"?
Google Translate has:
ביצה = egg, ovum, testicle, lump
I was told that ביעה is preferred to avoid the third meaning of ביצה.
Although Google Translate has "ביעה = A statement", when we say כביעה or מסכת ביעה the meaning is understood.
The Shaarei Teshuva on Orach Chaim 156 (2) quotes authorities that recommend beyim (plural of beyoh) instead of beitzim (plural of beitzoh). But he says that since nowadays the word beyoh has an association with male gonads it is fine to say beitzim. (I assume the same principle applies to the singular forms.)
Yam Shel Shlomo in chapter 4 of Bava Kamma says to pronounce it beya as the other word is not nice.
This is subsequently brought in the Magen Avraham in siman 156 which is probably where it picked up it's popularity.
Tiferes Yisroel in the beginning of the masechta named after the word in question, writes that he can't understand who wouldn't like the word beitzah which is found in the Torah. He suggests the source to pronounce it in the Aramaic way is based off the gemara where someone confused mei beitzim and mei bitza'im.
Someone once pointed out that Rabbeinu Channanel at the beginning of maseches Beitzah writes a rhyme which only works if you pronounce it beitzah.
אתחיל מסכת ביצה. בעזרת גדול העיצה.
It isn't clear to me how consistently he tried to rhyme his masechta opening lines, but this one seems quite intentional.
On a possibly related note, many rishonim would actually refer to this masechta as maseches Yom Tov.