Thomas H. B. Graham, Some English Expletives
“ A favourite one, that it represents a shortened form of the asseveration By Our Lady, is a very tempting one. It is, perhaps, as likely that the exclamation Blood! is a contraction of By our Lud as that it is the equivalent of the French Sang-dieu; and, by analogy, the oath By our leddy would naturally contract into blood! But the use of the word by itself as an interjection is so exceedingly rare that the above ingenious derivation of the term must, I am afraid, be abandoned. ”
