verglas

 vehr-GLAH
 A thin, transparent glaze of ice over rock. Extremely hazardous — crampons can't penetrate it and rock shoes can't grip it. The rock looks bare until you touch it and your hand slides.
Etymology
 French — verre (glass) + glas (ice), literally "glass ice." The word entered mountaineering English through Alpine climbing.
 climbing  ice/snow French mountain/alpine
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