Jennifer Aniston -- What's Sarong?

But soft! What light through yonder window breaks? It's Jennifer Aniston, who continued her ridiculously hot Mexico vacation yesterday ... looking pensive in a sexy bikini and an exotic red sarong (the thing wrapped around her waist).

As we reported, Jennifer and her tatted-up fiance Justin Theroux have been kicking it in Cabo San Lucas over Christmas, and things just keep heating up.

Still unclear if Jennifer's pregnant -- but if you're wondering what it's like to get held up in Mexico ... just ask her boobs.