hoar ‧ frost /ˈhɔːfrɒst $ ˈhɔːrfrɒːst/ BrE AmE noun [uncountable] formal
[ Date: 1200-1300 ; Origin: hoar 'gray, white' (11-20 centuries) (from Old English har ) + frost ]
a thin layer of ice that forms on objects outside when it is very cold SYN frost :
A light hoarfrost covered the fields.