Yeah, who knows the origins of slang...
I feel like many slang words are just shortened versions of the original word.
from the following, my guess would be that we go from bitcoin to bicoy(BiK-Oi) to something like Boy with a very gentle soft 'i' sound between the 'B' and the 'oi' sound, like Bi-Oi, most likely eventually resting at 'boy' without a strong 'y' at the end.
Taken from John McWhorter's Story to Human Language Course Guide
III. Typical sound change processes.
A. Assimilation. Many of these changes seem to us to be “sloppy”
speaking. For example, in early Latin, the word for impossible is
inpossibilis, but in later Latin, the word was impossibilis. The n
changed to an m because the m sound is closer to a p than n. This
process is called assimilation. Over time, laziness created a new
word—the one we borrowed from Latin that is so proper to us today!
in-possibilis > im-possibilis
B. Consonant weakening. Similarly, over time, consonants tend to weaken
and even disappear.
1. In Latin, the word for ripe was maturus. In Old Spanish, the word
was pronounced the way it is written today: maduro; the t
weakened into a d, and the s at the end vanished. But in Castillian
Spanish today, the word is actually pronounced “mathuro,” with
the soft kind of th in mother. In Old French, the word was similar,
pronounced “mathur,” but since then, the th sound has dropped out
completely, and the word is just mûr.