The term usually describes a requirement list that combines skills rarely found together, such as a senior engineer who also has deep clinical experience, speaks three languages and will accept a mid-level salary. Such roles can stay open for months while recruiters search for someone who may not exist. The pattern often starts at intake, when every wish from several stakeholders is added to the job description.
Chasing a purple squirrel lengthens time to fill and often ends with a compromise hire anyway. A better route is to separate true must-haves from preferences, ask which missing skills could be trained, and consider splitting the role. Screening against a realistic, weighted set of criteria then surfaces strong candidates who meet the core needs, rather than ranking everyone poorly against an impossible ideal.