Someone who gases, or something remarkably effective or impressive (slang); also a person who talks excessively.
From gas plus agent suffix -er. The slang sense of 'something impressive' emerged in 20th-century American English, while the talking sense is older.
Gasser as slang for 'something amazing' is purely American—in 1950s-60s slang, calling something a 'real gasser' meant it was hilarious or impressive, a meaning that's now almost forgotten.
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