Slang:

Slang is the use of highly informal words and expressions that are not considered standard in the speaker's dialect or language. It is very often specific to a particular context or group. It is notable for its liveliness, humour, emphasis, brevity, novelty, and exaggeration. Most slang is faddish and ephemeral, but some words are retained for long periods and eventually become part of the standard language. Slang is used for many purposes, but generally it expresses a certain emotional attitude; the same term may express diametrically opposed attitudes when used by different people. Slang
necessarily involves deviation from standard language, and tends to be very popular among adolescents. To one degree or another, however, it is used in all sectors of society.

Slang (Spanish)

Slang (English)

British Slang (English, with Phrases)

The Online Slang Dictionary (American, English, and Urban Slang)

A Dictionary of Slang (English slang and colloquialisms used in the United Kingdom)

Dictionary of Slang (La Mansión del Inglés)