Jennifer Aniston's Fiance Justin Theroux -- Bad Ass!

Who knew? Justin Theroux is tatted neck to toe.

The pics were snapped in Cabo San Lucas, Mexico over the weekend. Jennifer Aniston, who at 43 still looks mighty fine, and Justin were catching some rays when he stood up and revealed a back tat that looks kinda ... Asian-y.

Jennifer makes a yearly pilgrimage to Cabo around XMAS time and usually George Clooney follows suit -- this time with lucky Stacy Keibler. And Rande Gerber and Cindy Crawford always make the trek as well.

FYI --There are reports circulating that Jennifer is pregnant. If she's really pregnant, Jessica Simpson has been doing it seriously wrong.