Thrash

From WoWWiki

Thrash is a proc triggered ability which provides a chance to do extra attacks, similar to Windfury Weapon.

Boss ability

Some bosses have Thrash granting extra attacks, but because the combat log does not explicitly declare when a Thrash occurs, it is not a commonly-known boss mechanic and cannot be easily data-mined. The only indication of a boss having this ability is a series of hits all happening on the same time-stamp, which can only be seen by players with the right addons and UI configurations.

It is likely that Thrash (A) can proc from itself or (B) the number of extra attacks varies per boss, as players have witnessed a variety in the number of Thrash attacks coming from different bosses.

Items

There are several items with a built-in Thrash mechanic:

"Thrash" Items

Hand of Justice Binds when picked up Unique Trinket Requires Level 53 Equip: Chance on melee hit to gain 1 extra attack. Equip: Increases attack power by 20.	Flurry Axe Binds when equipped Axe One-Hand Speed 1.50 37 - 69 Damage (35.3 damage per second) Durability 105 / 105 Requires Level 42 Chance on hit: Grants 1 extra attack on your next swing	Thrash Blade Binds when picked up Sword One-Hand Speed 2.70 66 - 124 Damage (35.2 damage per second) Durability 90 / 90 Chance on hit: Grants an extra attack on your next swing.	Ironfoe Binds when picked up Unique Mace Main Hand Speed 2.4 73 - 136 Damage (43.54 damage per second) Durability 105 / 105 Requires Level 55 Chance on hit: Grants 2 extra attacks on your next swing.
Blinkstrike Binds when equipped Sword One-Hand Speed 2.60 147 - 275 Damage (81.2 damage per second) Durability 105 / 105 Requires Level 70 Chance On Hit: Grants an extra attack on your next swing.