Used at the end of a sentence to emphasize how you feel or think about something or what you are doing. Can be used as exaggeration or sarcasm as well.
That sciencetest was hard as sin.
I am tired as sin.
That girl was ugly as sin!
Mexican expression that is translated as "the cornerless" and used to refer to the anus. This definition is supported by the fact that the anus has no corners since it is round ( at least is not square) even if it has lines, dots or another characteristcs.