a common country practice was to catch strays and smear them with excrement*

AHD lists both (in separate entries) as Origin Unknown. This time I *will look it up in my OED!
And while you're at it Dr. Bill, you might consider expunging the word dirt from your vocabulary. Not to mention moist.


*According to ledasdottir, this would only work with bear excrement. (http://wordsmith.org/board/showthreaded.pl?Cat=&Board=miscellany&Number=32928)