Learn English – Origin and meaning of “damn straight”

The earliest reference I can find is from Hugh Wiley's first novel Wildcat, 1920:

The Wildcat, consuming a pork chop in the kitchen end of the mess hall, listened in. "Hot damn!" he exclaimed, "Grasty—was big words cooties, Honey Tone sho' would itch! Lissen at him go!"

A bit more about the book (from the Wiley link):

The Wildcat told the story of a black American drafted and sent overseas during World War I; several of Wiley's other early books, including The Prowler and Fo' Meals a Day (1927), were works depicting black life in comic and exaggerated manner, somewhat akin to minstrel show entertainment though perhaps a bit more subtle.

It comes via real-time strategy games, where one goal of the designers is to keep things balanced for all the various players; deviations from that goal are carefully scrutinized by the community that plays the game. More precisely, it comes from the regular complaint from certain types of gamers QQing over their losses — usually due to their inferior skill at the game — that the game's mechanics are broken or imbalanced.

Using imba, then, as a shortening of imbalanced, is a facetious way of mocking that attitude, by declaring even acknowledged essential parts of the game — such as the Hunter class in World of Warcraft, or some of its top players (usually insanely good ones) — imbalanced because in a way, if you don't know how to handle uber classes or uber players, they "break" the mechanics, albeit in a legal way.