In normal living things, pain functions as a warning system for the body that tells it to try and get away from whatever is causing the sensation. However, sometimes someone can't feel pain for whatever reason, usually due to nerve damage or other factors. Fiction likes to explore this idea and will often explore the consequences in a very philosophical way.
For humor purposes, a man may sometimes gain a temporary version when Distracted by the Sexy.
As in Real Life, a person like this is very likely to accidentally injure themselves and not realize it for some time. In fiction these people also tend to develop a skewed perception of reality and may take to killing people for fun. Other times they're simply shown as tragic individuals who are literally out of touch with the rest of us. note
It may become a Disability Superpower if the advantages are played up far more than the disadvantages. This is also a common feature for Super Soldiers, especially disposable ones. In such cases, the massive crippling drawbacks of not feeling pain tend to be downplayed, or, if addressed, aren't seen as a problem since they won't be alive long enough for them to have any effect.
It's also a common trait of androids and other robotic characters. As artificial constructs with computer "brains", there's no need for any warning system about damage they've received to actually be unpleasant, just like how real-life non-sapient computers don't feel pain from error reports. If an artificial lifeform can feel pain in the same way we understand it, that's a sure sign of Ridiculously Human Robots.
Contrast Frozen Face, also stemming from nerve damage.
See also Made of Iron and Nigh-Invulnerability for when a person resists damage, but may still feel something. This trope may result in a Major Injury Underreaction, though the latter is usually a temporary state of affairs. When a character with this trope is thought to be immune to emotional pain as well, they may eventually break down and ask Did You Think I Can't Feel?. When a character feels pain just fine but simply ignores it to keep going when a normal person would be debilitated, that's an example of Heroic Willpower and not this trope.
Can be a sub-trope to Master of Your Domain wherein the character is technically capable of feeling pain but can turn it off by an act of conscious will.
