An unwelcome guest is someone who is not invited or wanted in a particular place. The crossword solutions for this term are PEST (4 letters), BOOR (4 letters), INTRUDER (8 letters), CRASHER (7 letters), NON (3 letters), and GATECRASHER (11 letters). A PEST is someone who is annoying or troublesome, while a BOOR is someone who is rude and ill-mannered. An INTRUDER is someone who enters a place without permission, and a crasher is someone who attends an event without an invitation. Non is a French word meaning 'NO' or 'not, ' and a GATECRASHER is someone who enters a place without permission by bypassing security.