It's related to the modern English word "swear," and appears in a variety of words, including:
* Swear (verb): to make a solemn promise or declaration.
* Sworn (adjective): bound by oath or promise.
* Swore (past tense of swear): to have made a solemn promise or declaration.
* Forswear (verb): to renounce or abandon a belief, promise, or allegiance.
The SWER root is quite common in Old English, reflecting the importance of oaths and oaths in Anglo-Saxon society.