In the English language (and predominantly American English), it is relatively common to see a (limited) set of first and middle names that form initials used as a first name. (ex. TJ, JT, DJ). Thomas John Washington becomes TJ Washington. Where does this practice come from and why those handful of specific initials?
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I'm having a hard time finding sources for this, thus all of the following is from my own head.
A name like JT can come from having a given name they would rather not use, and a first + middle name that says JT, or JR, or anything else. Hyphenated first names aren't very common in English speaking countries but someone might also abbreviate something like Jean-Baptiste into JB.
If the second letter is J, in my experience it almost always refers to "junior". The reason someone might be called DJ is because they were named after their father named Douglas, David, whatever, and instead of others in the family calling both people the same name, they call one a shortened version. Maybe Doug, maybe David, but also maybe DJ (Doug Junior).