An ELL post says
I'd like to picture myself as a wealthy patron casually slipping the doorman a substantial tip as he doffs his cap / tugs his forelock and holds the door open for me to go through, thereby ensuring that I'll get attentive service from him for the duration of my stay.
which uses a pattern of "slip a tip".
I am aware that "tipping" means the customer gives $1 per bag to the doorman. I don't understand the meaning of "slip a tip". Could someone please give a hint?