The story
The exact origin of this suffix is unknown. Currently, it is used to bypass profanity filters, by the Crips street gang in the United States to avoid writing "ck" (as "ck" could stand for "Crip Killer"), emphasize the "k" sound, and sexualize certain words (e.g. thicc, the most popular word with this modification, or succ).
Etymology from Wiktionary, CC BY-SA 4.0.