Shit is a word considered vulgar and profane in Modern English. As a noun it refers to fecal matter, and as a verb it means to defecate; in the plural ("the shits") it means diarrhea. Shite is a common variant in British and Irish English. As a slang term, it has many meanings, including: nonsense, foolishness, something of little value or quality, trivial and usually boastful or inaccurate talk, or a contemptible person. It may also be used as an expression of annoyance, surprise, or anger.
Shit can comfortably stand in for the terms bad and anything in many instances (Dinner was good, but the movie was shit. You're all mad at me, but I didn't do shit!). A comparison can also be used, as in Those pants look like shit, or This stuff tastes like shit. Many usages are idiomatic. I'm shit out of luck usually refers to someone who is at the end of their wits or who has no remaining viable options. That little shit shot me in the ass, suggests a mischievous or contemptuous person. Euphemisms such as crap are not usually used in this context.
The term piece of shit is generally used to classify a product or service as being sufficiently below the writer's understanding of generally accepted quality standards to be of negligible and perhaps even negative value. The term piece of shit has greater precision than shit or shitty in that piece of shit identifies the low quality of a specific component or output of a process without applying a derogatory slant to the entire process. For example, if one said "The inner city youth orchestra has been a remarkably successful initiative in that it has kept young people off the streets after school and exposed them to culture and discipline, thereby improving their self esteem and future prospects. The fact that the orchestra's recent rendition of Tchaikovsky's Manfred Symphony in B minor was pretty much a piece of shit should not in any way detract from this." The substitution of shit or shitty for pretty much a piece of shit would imply irony and would therefore undermine the strength of the statement.
The term "I don't/doesn't give a shit" can be used when one does not care about something, or has a passive attitude toward said thing, as it denotes indifference. In context, one can say: "You're offended? Well, I don't give a shit!"
“You can’t polish shit” is a popular aphorism roughly equivalent to "putting Lipstick on a pig".