ST5 is a word used to substitute any word in the English dictionary. It can substitue a noun, a verb, an adjective, and so forth. Although ST5 can literally mean just about ANYTHING, it usually has a sexual innuendo. Also, ST5 usually has a negative connotation.
ST5 is also a tax exemption form.
"Wow, that dress you're wearing is so ST5!" -- replaces gorgeous or hideous
"Hey, gorgeous, do you want to go ST5 with me?" -- this can replace a various of verbs from grabbing coffees, dancing, and hooking up.
"You are such an ST5!" -- classic ST5 insult.