: A notification or sharing of old "combolists" from the massive Yahoo breaches (such as the 2013 or 2014 incidents which impacted billions of accounts).
: Use a reputable service like Have I Been Pwned to see if your Yahoo email address has appeared in any known "combolist" or data breach. Download 42x yahoo txt
The phrase is a specific technical shorthand frequently used in cybersecurity circles and data leak forums. It typically refers to a "combo list"—a text file containing a collection of usernames (or emails) and passwords—sourced from historical Yahoo data breaches. What is a "42x Yahoo" List? : A notification or sharing of old "combolists"