It is a term know within the European CS:GO community and derivates from "unlucky". It has been made popular by players like Lobanjica, Stark & JW ( steamcommunity.com/groups/Unlukers ) while streaming FPL (Faceit Pro League) Matches on Twitch.tv whenever a player messes up a round.
Adjective that describes a person/event subject to or marked by misfortune, particularly used in Wall-ball. Not producing the desired outcome; disappointing.
"Nahh that was below sea level, unlux Alexi" - Ms Wilton