Nethergarde Bitter in TBC Classic is a quest item for completing a quest at Honor Hold, here's where to get your hands on some Nethergarde Bitter.
Nethergarde Bitter TBC Classic: Where to Get Nethergarde Bitter
For completing the quest entitled "Gaining Mirren's Trust," you must bring Mirren Longbeard 1 Nethergarde Bitter, and have a Friendly reputation with Honor Hold.
This quest may be confusing because it sends you back to Azeroth into the Blasted Lands, the zone with the Dark Portal in it. Many players may be confused and searching around Honor Hold Keep, but you must go to Nethergarde Keep in the Blasted Lands.
Once you enter Nethergarde Keep, keep an eye out for a human with a purple shirt, he'll sell it to you.
As you have continued to quest in Hellfire Peninsula up to this point, you will have no problem having a Friendly reputation with Honor Hold. If you are a little short of it, keep doing other quests given to you from NPCs at Honor Hold until you do reach Friendly.