People from a variety of subcultures will often refer to themselves by adding "head" to the end of whatever they are interested in. For example, those interested in hip-hop often refer to themselves as "hip-hop heads", and those into sneakers as "sneaker heads", and those into marijuana as "potheads".
Does anyone know what the origin or first instance of this practice is? I'm sorry if this in not the correct sub-reddit to post this, but it seemed fitting and I have no idea of how I would go about researching that.
This is an etymology question. It's fine here, but you could try x-posting to /r/etymology. Meanwhile for quick reference, most good dictionaries include the etymology of words; there are also some specialist etymology dictionaries. Here's the Online Etymology Dictionary entry for head:
... Meaning "drug addict" (usually in a compound with the preferred drug as the first element) is from 1911. ...
As for pothead specifically
also pot-head "chronic marijuana user," 1967 ...