To kiss affectionately, especially in a prolonged or passionate manner.
Origin uncertain, possibly from dialectal German 'schmatzen' (to smack one's lips) or Yiddish 'schmues' (a chat, talk). Appeared in American English in the early 20th century. The onomatopoetic quality suggests it may have developed from the sound of kissing.
The word 'smooch' perfectly captures the playful, somewhat silly nature of affectionate kissing through its sound and informal tone. Its relatively recent appearance in English coincided with changing social attitudes toward public displays of affection in the early 1900s, when such casual terms became more socially acceptable.
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