mah phat willay

Basically, it's the way Shrek says "my fat willy" (aka: his big thicc green ogre hard-on)

Describes the length and thicc girth of one's dong. (picture a hockey puck, but longer, and thicker)

Shrek: "Aye Fiona, come souk mah phat willay."
Fiona: "Nah nigga u gay, go fuck Donkey."
Shrek: "...but his booner smells like ass"