For a very short time; lasting only a moment; also used in American English to mean 'in a moment' or 'very soon.'
From 'momentary' (Latin 'momentaneus,' from 'momentum' meaning a moment of importance) plus '-ly.' The meaning shift happened in American English in the 1900s.
This word is famously confusing because Americans and British people use it differently—British people say 'momentarily' means 'for a moment,' but Americans often say it means 'soon,' which bugs language purists constantly.
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