Used in fangirl circles as a cooler sounding variant to "sex." Can also mean: hot, sexy, cute, cool.

Other meanings vary on use. Be sure to check context!

Used most often in conjunction with "teh".

A combination of the words smut and sex. While sex can refer only to the mechanics of the act, the addition of smut into the equation (meaning to make obscene or pornographic) gives it a more vulgar connotation than just sex by itself.
This also applies to smexy. To be sexy is to arouse interest (even unintentionally), but to be smexy is to arouse sexual interest in a pointedly lewd manner.

word for sex on chat rooms that wont allow the word 'sex' and will ban you if you say it.

Smex is the kind of sex you read or watch when looking at or reading any kind of japanese cartoon. Is also used when read yaoi/yuri. Mostly used when reading yaoi/yuri, actually. "Smexy", a word favored by fangirls/boys everywhere, is used to describe any character he/she would like to throw down and screw their brains out...no matter who the character is really SUPPOSED to be with. For instance:

SMEX is NASA's abbreviation for, "Small Explorer". They use it in regards to missions using much smaller than normal shuttle-craft.

sex
with a 'm'
which stands for monkey
which led to the development of AIDS

so,
any sex in which you contract AIDS

A term for sex with a smelly person.