In Cannabis culture slang a roach is a term for the remainder of a joint

Joint (cannabis)

Joint is a slang term for a cigarette rolled using cannabis. Rolling papers are the most common rolling medium among industrialized countries, however brown paper, cigarettes with the tobacco removed, and newspaper are commonly used in developing countries. Modern papers are now made from a wide...

 or blunt

Blunt (cigar)

A blunt is a cigar which is wider than a cigarillo and not quite as wide as a Corona. These cigars typically consist of two main parts; the inner leaf, which is similar to a cigarette rolling paper, except it is made of tobacco, and a thicker outer leaf which is rolled around the inner leaf in a...

 after most has been smoked. Sometimes in the UK, Ireland and other European countries the term roach refers to the cardboard mouthpiece that is used at the end of the joint.

Etymology

The word "roach" entered American English from a Mexican Spanish slang word for marijuana, or a cigarette stub of marijuana, or tobacco adulterated with marijuana: cucaracha ("cockroach"; see also the folk song La Cucaracha

La Cucaracha

"La Cucaracha" is a traditional Spanish folk corrido that became popular in Mexico during the Mexican Revolution. It has additionally become a verse played on car horns.-Origins:...

.) In Spanish, refers to adulterated tobacco generally.

See also

• Cannabis smoking
  Cannabis smoking
  Cannabis smoking involves inhaling vapors released by heating the flowers and subtending leaves of the Cannabis plants, known as marijuana. Alternatively, the cannabis plant flowers may be finely sifted producing kief, a powder especially rich in the oil-glands or trichomes which contain the...