Where does “frost pocket” come from?

frost pocket (English) comes from English Frost, from Middle English frost, from Old English frost, from Proto-Indo-European prustós — frost.

Ancestry of “frost pocket”, step by step

frost pocket traces back through two separate lines of ancestry.

via English Frost

StepLanguageWordMeaning
1EnglishFrostA cover of minute ice crystals on objects that are exposed to the air. Frost is formed by the same process as dew, except that the temperature of the frosted object is below freezing
2Middle EnglishfrostCold, freezing, or frosty weather; weather that...
3Old Englishfrostproduced by metathesis
4Proto-Indo-Europeanprustósfrost

via English pocket

StepLanguageWordMeaning
1EnglishpocketA bag stitched to an item of clothing, used for...
2Middle Englishpocketbag, sack
3Anglo-Normanfro
Every word from Proto-Indo-European prustós