How did 'equity' semantically broaden to mean 'common shares'?
Equity — not to be confused with equity in real estate — is another word for stocks.
early 14c., "quality of being equal or fair, impartiality in dealing with others," from Old French equite (13c.),
from Latin aequitatem (nominative aequitas) "equality, uniformity, conformity, symmetry; fairness, equal rights; kindness, moderation,"
from aequus "even, just, equal" (see equal (adj.)).
As the name of a system of law, 1590s, from Roman naturalis aequitas, the general principles of justice which corrected or supplemented the legal codes.