Frostiness is a quality of being very cold and icy. You might note the frostiness of your lawn one early morning in November and realize you should go back and grab your winter coat. Brrrr!
The frostiness of a snowy field or a vanilla milkshake is all about their temperature, and specifically about frost — the frozen white stuff, made of ice crystals, that forms on something when it's very cold. There's also a figurative type of frostiness, a manner of interacting with others that is aloof, cold, and distant: "His frostiness made me think he wouldn't be a great kindergarten teacher."
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