A judge, especially in the Isle of Man; a judicial officer who makes legal decisions.
From 'deem' (to judge) plus the suffix '-ster' (one who does something). The term is particularly associated with the Isle of Man's judicial system, where deemsters have been judges since medieval times.
Deemsters are still active judges in the Isle of Man, making this one of the few archaic-sounding words that's still a living job title—you can actually become a deemster today, which is pretty cool for a word that sounds like it belongs in a fantasy novel.
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