Characteristic of or resembling December, such as having winter weather, darkness, or holiday atmosphere.
From December + '-ly' (a suffix forming adjectives). This adjectival form is rarer than 'decemberish' but follows the standard English pattern of adding '-ly' to month names.
English has different ways to describe seasonal qualities: November → novemberish/decemberly, but we rarely use these forms because we prefer concrete words like 'wintry' or 'autumnal' instead.
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