fanny
Explanation
This comic features Batman (shown in silhouette with his characteristic cape and cowl) being confronted by a passerby who says "Cool fanny packs!" Batman indignantly responds that they are NOT fanny packs -- they're turned to the side, and fanny packs are worn in front. The passerby responds with a sarcastic "Riiiight."
The joke is that Batman's iconic utility belt -- the signature accessory that holds all of his crime-fighting gadgets and is central to his tough, intimidating image -- looks a lot like a fanny pack. Batman's defensive insistence that the positioning is different (side vs. front) only makes the comparison more embarrassing, as he's essentially admitting the only difference between his high-tech crime-fighting gear and a dorky tourist accessory is the angle at which it's worn.
The caption -- "This is the quickest way to ruin Batman's day" -- underscores that for all the villains and threats Batman faces, the most devastating attack is simply pointing out that his utility belt is basically a fanny pack.