tell sb to do sth (Cambridge Learner's Dictionary)
When I look in dictionaries, I often see the words sb and sth. Are these proper words? What do they mean?
Can I use these words in my essays, for example?
Can I use these words in my posts on Stack Exchange?
Best Answer
sb is an abbreviation for somebody. sth is an abbreviation for something.
The sentence means tell somebody to do something. In real life, any person and any thing can be included in the sentence, for example, tell [your student] to [complete their homework]. A dictionary should explain this.
Can you use these words in your essays? No. They are not real English words.
Can you use these words in your posts on Stack Exchange? Maybe. Until you are very certain how to use them, use the words in full.