Alexander Dyce, A General Glossary to Shakespeare's Works

bilboes — “The,” HAMLET, v. 2. 6. “The bilboes is a bar of iron with fetters annexed to it, by which mutinous or disorderly sailors were anciently linked together. The word is derived from Bilboa, a place in Spain where instruments of steel were fabricated in the utmost perfection. To understand Shakespeare's allusion completely, it should be known that, as these fetters connect the legs of the offenders very close together, their attempts to rest must be as fruitless as those of Hamlet, in whose mind there was a kind of fighting that would not let him sleep. Every motion of one must disturb his partner in confinement. The bilboes are still shown in the Tower of London, among the other spoils of the Spanish Armada” (STEEVENS) .