Something or someone that produces or generates heat.
Compound of 'heat' and 'maker' (from Old English 'macian' meaning to make or construct). This transparent compound formation is productive in English for creating agent nouns describing what something does.
Compound words like 'heatmaker' are economical—instead of saying 'something that makes heat,' we compress it into one word. This same logic gave us 'peacemaker,' 'troublemaker,' and 'pacemaker,' all following the same pattern.
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