A person who dives underwater using an aqualung or similar breathing apparatus.
From 'aqualung' plus the agent suffix '-er' (Old English -ere), which transforms a tool or device into the person who uses it, similar to 'swimmer' from 'swim' or 'surfer' from 'surf'.
The term 'aqualunger' is charmingly specific—it refers to someone equipped with that particular 1940s technology, whereas today we'd say 'diver' or 'scuba diver,' showing how language evolves as technology becomes commonplace.
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