I’m very disappointed in poem_for_your_sprog for almost understanding this point, and then immediately making the same mistake she was calling out
‘I have to sort my books!’ she cried,
With self-indulgent glee;
With senseless,narcissisticpride:
‘I’m just so OCD!’
‘How random, guys!’ I smiled and said,
Then left without a peep -
And washed my hands until they bled,
And cried myself to sleep.
Thought it originated as being slang for an undercover narcotics officer. So an undercover cop who busts you for drugs. Then by extension, an informant who rats you out to such an officer.