I was doing some searches on Google Books tonight using "Jehovah's witnesses" and I was stunned with these references from 19th and early 20th century texts which use the term "Jehovah's witnesses." I don't have the time to develop this info into a full-fledged post, but I ask those who might be interested in the subject to check these references out. (I realize some of these may not work outside the US due to copyright issues and I've put the page number next to the URL to aid in finding the reference). All of these are from works way before Rutherford got the idea (supposedly by divine guidance) in 1931:
http://books.google.com/books?id=-MxCAAAAIAAJ&pg=PA313&dq=jehovah%27s+witnesses&lr=&as_brr=1
Page 313
http://books.google.com/books?id=_18NAAAAIAAJ&pg=PA350&dq=jehovah%27s+witnesses&lr=&as_brr=1
Page 350
http://books.google.com/books?id=cWVCAAAAIAAJ&pg=PA286&dq=jehovah%27s+witnesses&lr=&as_brr=1
Page 286
http://books.google.com/books?id=wJgAAAAAMAAJ&pg=RA6-PA442&dq=jehovah%27s+witnesses&lr=&as_brr=1
Page 442
http://books.google.com/books?id=rVYyrpDvQtIC&pg=RA4-PA411&dq=jehovah%27s+witnesses&lr=&as_brr=1
Page 411
http://books.google.com/books?id=jYFJAAAAMAAJ&pg=PA95&dq=jehovah%27s+witnesses&lr=&as_brr=1#PPA97,M1
Page 95
http://books.google.com/books?id=cQk3AAAAMAAJ&pg=PA228&dq=jehovah%27s+witnesses&lr=&as_brr=1
Page 228
http://books.google.com/books?id=IDM2AAAAMAAJ&pg=PA17&dq=jehovah%27s+witnesses&lr=&as_brr=1
Page 17
http://books.google.com/books?id=jf42AAAAMAAJ&pg=PA49&dq=jehovah%27s+witnesses&lr=&as_brr=1
Page 49
http://books.google.com/books?id=hWkBAAAAQAAJ&pg=PA82&dq=jehovah%27s+witnesses&lr=&as_brr=1
Page 82
Many of these are from scholarly texts. They are not obscure writings. One of these is from Bible commentator Dummelow who was quite well known. Another is from a text published by Charles Scribner & Sons, which published scholarly religious works. Apparently, it was commonplace for Bible scholars of that era to refer to the people of Israel as "Jehovah's witnesses." (Many scholarly works then used "Jehovah" just as many now use "Yahweh".)
It may be premature to say Rutheford borrowed the idea. Maybe not. But, we can say for sure the name "Jehovah's witnesses" was far from unique.