A viral theory claims "luck" truncates Lucifer, so wishing someone luck invokes the fallen angel. Circulates widely in religious conspiracy content.
Assessment: Linguistically impossible. The words share no root; luck entered English from Germanic gambling slang a millennium after Lucifer was named in Latin scripture.
Documented
A gambler's word
Likely entered 15th-century English from Low German or Dutch luk, a shortening of geluk, used in gambling. Its earliest English uses are games of chance.
Assessment: Documented in the historical record.
Stories
The rituals outlived the word
Horseshoes over doors, wood knocked on, salt thrown over the left shoulder, sailors refusing certain words at sea. The story tier of luck is not etymology at all: it is the universal human bet that the universe responds to gestures.
Assessment: As documented folklore, all real and ancient. As evidence about the universe, it is the longest-running experiment with no results, and the participants refuse to cancel it.
Sources and further reading
OED, luck, late Middle English from Middle Dutch luc and Middle High German gelücke
Worth asking
Why do humans prefer a hidden name over a gambling word as the source of fortune?