松 (sōng) as an adjective means 'loose' (not tight) for physical objects like belts or knots; as a verb it means 'to loosen' or 'to relax'. 放松 (fàng sōng) specifically means 'to relax' (one's body or mind) and is the common verb for that sense, while 松 alone is more often used for physical loosening. 宽 (kuān) means 'wide' or 'broad' (not tight in fit), not 'loose' in the sense of slack.