I had look through the dictionary and found there are short form for “somebody” and “something”, which are “SB” and “sth”. However I couldn’t find a short form for “somewhere”. Is there a short form for “somewhere”?
The abbreviations sb and sth are only used in dictionaries. They are never used in general writing.
Dictionaries need to save space. They use "sth" and "sb/sby" as a short way of writing these words which are frequently used in definitions.
open (sth): cause (sth) to open
As dictionaries move online, some are now not using these abbreviations.
But learners tend to use dictionaries more often than native speakers, and might think that "sth" and "sb" are in general use. They are not! You would not write:
Sb left a chocolate on my desk today!
Not even as a casual post on Twitter.
So unless you are writing a dictionary, don't use sby or sth. And don't use an abbreviation for somewhere.