Past tense of 'guest' (as a verb); appeared as a guest, especially in entertainment or on a show.
From 'guest' as a verb (to act as a guest or to appear as a guest performer) plus regular '-ed' past tense. The verb 'to guest' is relatively modern, emerging prominently in entertainment contexts.
Verbing nouns—turning 'guest' into a verb—is a constant feature of English; advertisers and media popularized 'to guest' (as in 'to guest on a talk show'), demonstrating how English effortlessly converts word classes depending on communicative needs.
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