In a crooning manner; while crooning or in a soft, sentimental, singing way.
From 'crooning' plus '-ly' adverbial suffix. Allows the adjective quality of crooning to modify verbs and other actions.
Adverbs ending in '-ingly' are increasingly rare in modern English—we say 'she sang crooningly' less than we used to, which mirrors how the crooning style itself has faded from popular music.
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