Etymology: Middle English, from Old English sceard; akin to Old English scieran to cut -- more at SHEAR
1 a : a piece or fragment of a brittle substance; broadly : a small piece or part b : SHELL, SCALE; especially : ELYTRON
2 usually sherd : fragments of pottery vessels found on sites and in refuse deposits where pottery-making peoples have lived
3 : highly angular curved glass fragments of tuffaceous sediments

Slang name commonly used to describe a piece of crystal methamphetamine or meth.

A piece of crystal meth, a psychedelic stimulant, named for the shard-like crystals. Has nothing to do with shitting in your pants while farting, that's shart.

The act of farting in your pants, and it seems to spray your underwear like a spraypaint can.

A little bit Shit and a little bit fart.

A large crystal like rock, usually bigger than a piece of rice is known as a shard.

A portmanteau of the verbs "shit" and "fart," which describes the instance of accidentally shitting one's pants based on the assumption of farting prior to release.

A bit of technical information:

Despite one's intuition that the un-inflected form of the verb would be "shart," (borrowing the onset from "shit" and the rhyme of "fart") It's phonological realization, "shard," implies that the word entered the lexicon as "sharted."

Much in the same way the words "writer" and "rider" can not be distinguished from one another in speech with out a rich enough context (because they sound the same), "sharded" and "sharted" will also sound identical due to flapping. Hence, speakers of English chose "shard" as opposed to "shart" by means of back-formation. Selection of the phoneme /d/ over /t/ can likely be attributed to the feature {+voice} of the flapped consonant.