The word “yoke” derives from the Latin word iugum, a collar used to join a pair of oxen. The verb, subjugate, originally meant to place animals under a yoke, but today we only use the word metaphorically when people are forcefully put under someone’s rule. When a Roman army conquered a new territory, they made the inhabitants march under a symbolic yoke to drive home the message that they were now “subjugated”, that is, under the yoke of Rome. Here, Cassius is using the noun, yoke, metaphorically. The Romans are groaning under the yoke Caesar has placed on them, just like oxen laboring under their yoke.