Uncertain. Probably from Latin solidus (“gold coin”) (contracted to soldus in Late Latin- compare Italian and Portuguese soldo, Spanish sueldo, French sou, etc.), through an early Romanian root *soldu and its plural *solḑi. Thus possibly a doublet of solid. Other theories suggest Slavic *sъlzъ, but this is less likely. Also possibly an expressive creation.
solz m (plural solzi)