Someone who judges, considers, or thinks about something; a person who forms opinions or assessments.
From the verb 'deem' (Old English 'deman' meaning to judge or decree) plus the suffix '-er' indicating an agent who performs the action. The word reflects how 'deem' emphasized judicial thinking.
A deemer is basically anyone who judges—but the word itself is archaic enough that using it makes you sound like you're pronouncing judgment from on high, which is probably why you hardly ever hear it anymore despite it being perfectly logical.
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