The Giddy Goop is a tier 2 trinket that is obtained with a 1% chance from normal Fireflies or a 1/15 chance from stealing.
When equipped, it makes the finishing move of any weapons combo fire a goop that deals 25 damage. This means it does not work on attacks that don't have combos, such as fists, ranged weapons, or Acorn Turrets. It does not work with all Daggers either, due to them having a quicker combo that consists of only 2 hits instead of 3.
This goop, along with dealing damage, will also inflict -Movement Speed and -Attack Speed to any Creatures it hits.