Unmarked by lines, especially of the skin(adjective)
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Use "unlined" in a sentence
"His cheeks are full and his skin unlined; he looks far younger than his 67 years."
"Plus, electronics sometimes wind up sitting around in unlined pits, where chemicals can leech into soil and groundwater, poisoning the surrounding areas."
"McGrath points out that the disposal site is in a 100 year floodplain and is "unlined," which allows toxins from the coal ash to leak into the area's groundwater and potentially into someone's drinking water downriver."