Thoughtless describes someone who does not stop to think how their action will land on other people. They leave the last of the milk out on the counter, they play music loudly at midnight, they talk about a party in front of the one friend who was not invited.

The key thing — and it is the thing students most often miss — is that thoughtless is not the same as cruel. A thoughtless person is not trying to hurt you; that is exactly the point. Their mind was full of themselves and simply had no room for you in it. That is why 'thoughtless' is a favourite word for gentle criticism between people who like each other: 'That was thoughtless of me, sorry.' Calling an act cruel ends a friendship; calling it thoughtless usually repairs one.

There is a second, more everyday use with no unkindness in it at all: done without thinking, careless. 'A thoughtless mistake' just means one made on autopilot. The context tells you which sense you are in — if there is a victim, it is the unkind sense.

Be careful with the shape of the word. It looks like the opposite of 'thoughtful', and in the being-kind sense it is. But 'thoughtful' has a second life meaning deep or reflective ('a thoughtful essay'), and 'thoughtless' does NOT reach into that meaning — the opposite of a thoughtful essay is a shallow one, never a thoughtless one.

Grammar note: it describes the person or the act, and the common frame is 'It was thoughtless of him to…'. Say 'thoughtless of', not 'thoughtless to' someone.