Sneaking it in means: This technique is used when a girl will let you touch her but not allow you to have anal sex. It is most effective when she’s curious about it, and her naughty thoughts are turning on. After you’ve successfully poked one and then two fingers into her genitals, it is possible to sneak it in. You quickly switch between the two hands, taking the fingers from her and slipping your cock inside. She will likely accept the fact that there is a cock in her genitals, and continue to take it. (in Community Dictionary, added by Jazlynn Bush)