Pitiless means "without mercy" or "cruel." Movie critics are often described as pitiless, because if they hate a film, they don't hold back or consider the feelings of the people who poured their hearts into making it.
The root word of the adjective pitiless is pity. If someone lacks all sympathy or tenderness, you can describe them as pitiless. People who are annoyed that they have to step out of the way of homeless people who huddle against buildings in winter are probably pitiless — they just don't care about the plights of others. How can you tell if your friends are pitiless? They seem bored or annoyed when you are upset about something.