tk · adjective1
Abbreviation of to come. C is replaced with K because -tk- occurs much less often in English than -tc-, making it easier to search for in a text.
TK · adjective, noun2
Phonetic initialism, with "TK" chosen over "TC" because scarcely any English words contain the sequence "tk," making it easily searchable.
Etymology from Wiktionary, CC BY-SA 4.0.