The story
Abbreviated from the year 2000, where K denotes 1,000, derived from the prefix “kilo-.” However, according to the International System of Units (SI), the correct prefix for 1,000 is the lowercase k. The uppercase K is sometimes used informally, particularly in computing contexts, where it may denote 1,024 rather than 1,000.
Etymology from Wiktionary, CC BY-SA 4.0.