a robber who takes by open force: a violent bandit or highwayman, and by extension an armed rebel.
A robber of this kind works in the open, by force. Picture a highwayman, a raider, an armed brigand who overpowers travelers and plunders them, not a burglar who sneaks in unseen. That quiet, stealthy stealing is a different Greek word (κλέπτης); this one carries danger and violence with it. When the man in Jesus' story falls among robbers, they beat him, strip him, and leave him half dead.
The word also had a sharp political edge in Jesus' day. The authorities used it for armed rebels and revolutionaries who took up weapons against Rome, so calling someone a robber could brand him an insurrectionist. Barabbas wears this label, and so do the two men crucified beside Jesus, who died the death Rome reserved for sedition. The result is a word that sits in a charged zone between plain banditry and open revolt.
Jesus also turns the word into a picture. In his shepherd teaching, the one who climbs into the sheepfold by another way rather than the gate is a robber, a predator who comes only to plunder the flock, set against the true shepherd who lays down his life. In the same spirit he charges that the temple, meant to be a house of prayer, has been made a robbers' den, a safe hideout for the corrupt.