On a swing set, where the person goes under the swing and then lifts the swing, typically what moms, dads and babysitters do to toddlers. But i've seen teenagers challanging each other by making there friends give them underducks.
i remember 7 years ago, when i was little my mom gave me an underduck, and In the GIF i attached it shows exactly with an underduck looks like
by username420 December 04, 2020
When you bend your dick down and back, and a girl blows you from behind with her nose on your butt hole.
by TurdFerguson21 April 20, 2016