Answer:
Thymus
Lymphocytes could be roughly divided into two categories. B cell and T cell.
Though they both originate from the bone marrow, T cell migrate to the" thymus," where it reach maturity while B cell got mature in the bone.
That is, T stands for thymus.
However, B doesn't mean "bone" actually, for B cell was first discovered in the "bursa of Fabricius" in a bird.