Covered partially by shadow so it's darker; or the past tense of following someone closely and secretly.
From Old English 'sceaduwe' meaning shade or shadow. The verb 'shadow' meaning to follow was an extension of the literal meaning, as if you were following close as a person's shadow follows them, which emerged around the 15th century.
When detectives or security personnel 'shadow' someone, they're literally using a metaphor thousands of years old—you can't escape your shadow, so the word perfectly captures being closely followed and unable to shake someone off!
Complete word intelligence in one call. Free tier — 50 lookups/day.