In a recent review of Roger Hart's superb Chinese Roots of Linear Algebra (2010) I've described what I believe is the motivation of Donald Knuth's symbol Θ ("big theta") in asymptotic notation.
I think it is a graphic pun on the O ("big O") that stands for German Ordnung 'order', as in "order of growth". I suggest that the bar in the middle of Θ is intended to represent tight bounds above and below, in contrast to big O (upper bound) and big omega (lower bound). I have not seen this explanation anywhere else.
Hart's book is presently described on the Johns Hopkins University Press site, though such listings tend not to be permanent. The Project MUSE page for his book is at https://muse.jhu.edu/books/9780801899584.
For more on Hart's research, see his website, http://rhart.org/.