I thought it was a clarifier; most geese are known to present aggressively, so you're referencing an anomaly. In this case, calling one a 'silly goose' would extrapolate to, "You're unusual, possibly judgementally-impaired, but not in a way I find detrimental to anyone but yourself so I'll make an light-hearted offhand comment, rather than a direct and serious criticism More @Wikipedia
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