Today’s One Big Happy, in which it turns out that Ruthie isn’t the only character who’s unsure about word meanings:
NOAD2 identifies gormless as informal and specifically British, so it’s no surprise that the adults don’t know what it means (though the appalling Avis takes it back to a putative noun stem gorm, which she treats as a mass noun (gormless ‘without gorm, lacking gorm’), though it could be a count noun (gormless ‘without gorms, lacking gorms’)).
The rest of the NOAD2 entry:
lacking sense or initiative; foolish. a constantly grinning, rather gormless boy.
ORIGIN mid 18th cent. (originally as gaumless): from dialect gaum ‘understanding’ (from Old Norse gaumr ‘care, heed’) + –less.
Presumably the base word was something like [ gɔ:m ] (British English being predominantly non-rhotic), which is interpretable as lexically / gɔrm /, with the postvocalic r vocalized. (Puzzled British student to linguistics instructor: “Why do Americans have an r [ ɑ: ] in dog [dɔ:g ]?” — interpreting the AmE pronunciation as lexically / dɔrg /. )
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