Warlock
"A man practicing the black arts"

Its roots lie in Middle English warloghe, which comes from Old English wǣrloga, meaning "one that breaks the faith". Wǣr is faith, and loga comes from lēogan which is to lie.

So a warlock implies that they're someone who deals with lies and "dark spirits", they've broken with the faith (thus they're heretics). Overall, a rather not-nice person from the view of society.

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For Sorcerer we need to look to Sorcery to find the meaning.
Sorcery
"The use of power gained from the assistance or control of evil spirits especially for divining"

A Sorcerer is one who practices Sorcery, meaning someone who has power from the assistance and/or control of evil spirits.

The origins here come from Middle English sorcerie, with roots to Anglo-French and Medieval Latin. From Medieval Latin we have sortarius (from sort; chance, lot). Now I don't really know Medieval Latin, but from what I can understand, Sort+arius would be something like "someone connected to chance".

So similarly to a Warlock, a Sorcerer implies someone dealing with "bad stuff", in this case evil spirits. I would guess *maybe* the reason for choosing Sorcerer as the name for the spontaneous casting class was because of the more "chance"-aspect of how a sorcerer gets their power in the D&D-fluff.


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Wizard
"A wise man"

A wizard is a wise man, a sage, one who has magical influence or power.

The etymology of Wizard, compared to the two above, is easier. It comes from the Middle English wysard, from wis/wys that means wise. So meaning something like "someone who is wise" or "wiseman".

Wizard is synonymous with Sorcerer, but not with Warlock, which is interesting. Wizard being the "power through long study" works well with its roots in the word "wise" as it's implied that someone who has gained a lot of knowledge is a wise person (but not nescessarily someone with a high Wisdom score ).