1. Without a basis in reason or fact
   "the allegations proved groundless"
   - baseless, idle, unfounded, unwarranted, wild

See also: unsupported

• unsweet
• unsweetened
• unswept
• unswerving
• unswervingly
• unsworn
• unsyllabic
• unsyllabled
• unsymbolic
• unsymmetric
• unsymmetrical
• unsymmetrically
• unsympathetic
• unsympathetically
• unsympathising
• unsympathizing
• unsynchronised
• unsynchronized
• unsynchronous
• exorciser
• exorcism
• exorcist
• exorcize
• exordial
• exordium