Coincidental means happening together purely by chance, not by design. No connection, however it looks.
The phrase any resemblance is purely coincidental is the legal formula printed at the front of novels and films to head off complaints, and it is where most people meet the word. It matters in argument, since two things happening together does not make one cause the other, and dismissing a pattern as coincidental is sometimes right and sometimes exactly what somebody wants you to believe.
1adjectivecoincidentalResulting from chance rather than from any connection.
“The timing was entirely coincidental.”
Also accepted in games: accidental
From co- + incidental.
Etymology from Wiktionary, CC BY-SA