This is the 9136th most frequent Russian word.
свинцов
"Свинцов" means "lead (made of or related to the metal lead)."
In this sentence, 'свинцовый' is used as an adjective describing the material of the object, specifically 'lead', implying that the weight was made from or contained lead.
Свинцовый груз был слишком тяжелым.
The lead weight was too heavy.
Here, 'свинцовые' describes the appearance of the clouds, likening their color and heaviness to that of lead, creating a metaphorical expression.
Свинцовые тучи закрыли небо.
Lead-colored clouds covered the sky.
In this usage, 'свинцово' is an adverb derived from 'свинцовый', used metaphorically to express a heavy or gloomy tone, akin to the properties of lead.