fly block

(in a Spanish burton or the like) a block, supported by a runner, through which the hauling part of the fall is rove.

any block that shifts with the movement of its tackle.

Origin of fly block

First recorded in 1835–45

The block secured to the lower end of the tye is the fly-block.

The Sailor's Word-Book |William Henry Smyth
The halyards should be a luff purchase, the fly-block being the double block, and the single block being hooked in the channels.

The Seaman's Friend |Richard Henry Dana
Reeve the end of your rope through the upper block, and make it fast to the strap of the fly-block.

The Seaman's Friend |Richard Henry Dana
Have the fly-block of the topsail halyards above the top, and rack the runner to the topmast backstay or after shroud.

The Seaman's Friend |Richard Henry Dana

Then make fast your hook to the bight of the rope, and reeve the other end through the fly-block for a fall.

The Seaman's Friend |Richard Henry Dana