If you already know what "THAC0" stands for, then you probably already know what I'm going to say in this post as you've probably got analogous experiences to me!
In Dungeons and Dragons, the chance to hit (and indeed: to wound) an enemy was determined by the THAC0. In short, it stands for "To Hit Armour Class 0". Every player character (and opponents!) had a numerical (integer) THAC0 value that started out at a modestly high level for lower level characters, and got lower as the player character increased in level.
In turn, everyone also had an Armour Class. The better the armour that someone was wearing, the lower that armour value would be. It even extended to negative values.
Let's take an example to make this concept a bit more obvious. A level 5 warrior has a THAC0 of 16. That means that if he (or she) is facing an opponent with an armour class of 0, they need to roll 16 or more on a d20 to hit and wound that enemy. If the opponent was wearing slightly worse armour, they might have an armour class of 10 instead. Therefore, the warrior receives a modifier of 10 to this. Hence they'd need a 6 or more to damage them. And if their armour was superb, they might have an armour class of -2. That would need a roll of 18 or more on a d20 to do damage. And so forth.
THAC0 was eliminated from Dungeons and Dragons with the advent of 3rd edition (in the year 2000). Armour classes were reversed such that higher values of armour were better (rather than lower). Yet, the concept of THAC0 and its abbreviation will (I'm assuming!) sit strongly in people's minds if they've played Dungeons and Dragons in yesterdecades.