1857. Snowden, Mag. Assistant, 3 ed., p. 444. To inform = to come
it.
1864. Hotten, Slang Dict., p. 126. The expression come it (to
inform, tell, or disclose) is best known to the lower and most
dangerous classes.
1889. Daily Telegraph. He heard one of the others say in reply,
‘come it,’ meaning to tell—to be quiet.