To indulge in fantasy; to imagine things only possible in fantasy.(verb)
To portray in the mind, using fantasy.(verb)
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Use "fantasise" in a sentence
"But he added that some female students still "fantasise" about lecturers."
""It is evidently pleasing to many people," the demographer J Richard Udry observed dryly in a 1970 paper comprehensively disproving the theory, "to fantasise that when people are trapped by some immobilising event which deprives them of their usual activities, most will turn to copulation"."
"The only time I fantasise about stuff like that, is when I pair them up with ME!!"