What were the Eunuchs exactly!?

[Nathan Ross]

that's the origin of the saying "cum pedibus albis", it most likely denotes foreign origin

Found it. The reference is Pliny, Natural History 35.38:

"...the most inferior kind of chalk; which was used by the ancients... for marking the feet of slaves on sale, that were brought from beyond sea..."

Pliny goes on to say that some of these slaves attained high rank, and were "sent back in state to the very place from which they originally came, with their feet whitened with the slave-dealer's chalk!"