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?
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.
Dictionaries traditionally use a lot of abbreviations. This is because, like everything else, they used to just be printed volumes, not online resources, and they were trying to cram as much information into a small a space as possible. Thus, they developed abbreviations for parts of speech, like v. for verb, for characteristics of different words, like trans. for transitive. They also had abbreviations for placeholders that would be needed when describing different uses or senses of words, like sb for somebody and sth for something, so you can give definitions related to specific usages like "to give sb five" for the now rather dated gesture of slapping palms with someone.
There was, as I mentioned, a good reason for dictionaries to use these. There isn't a good reason to use them generally. Sure, use them yourself in your own notes for brevity - or because you don't want to write things out in full if abbreviations are good enough for you to understand your own notes; everyone should make notes however works best for them. They are, I understand, used other than in dictionaries just as a way to save space and effort. But if you're communicating with other people, people with whom you do not have a shared vocabulary of abbreviations that includes these, the risks of not being understood (and just seeming weird) are far greater than any possible benefit.
I do not have printed (paper) dictionaries, so I use the on-line versions of the dictionaries.
sb in the Cambridge Dictionary:
sb = written abbreviation for somebody or someone
sth in the Cambridge Dictionary
sth = written abbreviation for something
Of course, several words / expressions can be abbreviated as sb or sth.
E.g.:
Sb = stibium (antimonium)
SB = Sound Blaster
I found all that with a simple Google search. All information was on the first page of results. I will not copy / paste all definitions from all dictionaries, as it is not the point.
Are these proper words?
Hell, no! :) They are abbreviations, of course (I hope).
I already asked a specific question here.
Can I use these words in my essays, for example?
As far as I know, academic papers, like any formal communication, should use the minimum amount of abbreviation. Of course, some "famous" abbreviations are allowed (etc., e.g., i.e., names of institutions...), but other than that, not really.
Can I use these words in my posts on Stack Exchange?
Stack Exchange is a (mostly) informal place of communication, so abbreviations are not forbidden. However, their use can be ambiguous, and they should either be avoided, or disambiguated (defined).
Example: one question was about PC. The first words that popped into my head when I saw that, were "Personal Computer". However, OP meant "Politically Correct".
