An idiolect is a language the linguistic (i. e. syntactic, phonological, referential, etc.) properties of which can be exhaustively specified in terms of the intrinsic properties of some single individual, the person whose idiolect it is. The force of “intrinsic” is to exclude essential reference to features of the person’s wider environment, and in particular to their linguistic community. (Stanford Encyclopedia of Philosophy)

Variety of a language used by a group of speakers within a particular speech community. Every individual speaks a variety of his language, termed an idiolect. (Source)

A dialect is a version of a language spoken by a group of people. An idiolect is much smaller — it’s the way a particular person speaks, at a specific time, as distinct from others. This word is mainly used by linguists when discussing differences in speech from one person to another. Like your fingerprint, your idiolect is unique. It’s kind of like a micro-dialect. (Source)

William Archibald Spooner (22 July 1844 – 29 August 1930) was a long-serving Oxford don, notable for absent-mindedness, and supposedly liable to mix up the syllables in a spoken phrase, with unintentionally comic effect. Such phrases became known as spoonerisms, and are often used humorously. Many spoonerisms have been invented and attributed to Spooner. (Source: Wikipedia)

A spoonerism is an error in speech in which corresponding consonants, vowels, or morphemes are switched (see Metathesis) between two words in a phrase.^[1][2] These are named after the Oxford don and ordained minister William Archibald Spooner, who was famous for doing this.

An example is saying "The Lord is a shoving leopard" instead of "The Lord is a loving shepherd." While spoonerisms are commonly heard as slips of the tongue, and getting one's words in a tangle, they can also be used intentionally as a play on words. (Source: Wikipedia)

Examples of spoonerisms

Idiolects versus Social Languages

Key to the distinction between an idiolect and a social language is the fact that the same natural language, L, can be picked out in either of two ways: L as the language with specified linguistic (semantic, syntactic, phonological, etc.) properties, or L as the language possessed (spoken, etc.) by a specific individual or population. French, for example, is the language containing an adjective, rouge, that refers to the color red and in its spoken form begins with a voiced uvular fricative, etc. But French is also the first language of most residents of France, the lingua franca of Côte d’Ivoire, the medium in which Simone de Beauvoir wrote, spoke, and thought, etc. (Stanford Encyclopedia of Philosophy)

Geographic dialects

The most widespread type of dialectal differentiation is regional, or geographic. As a rule, the speech of one locality differs at least slightly from that of any other place. Differences between neighbouring local dialects are usually small, but, in traveling farther in the same direction, differences accumulate. Every dialectal feature has its own boundary line, called an isogloss (or sometimes heterogloss). Isoglosses of various linguistic phenomena rarely coincide completely, and by crossing and interweaving they constitute intricate patterns on dialect maps. Frequently, however, several isoglosses are grouped approximately together into a bundle of isoglosses. This grouping is caused either by geographic obstacles that arrest the diffusion of a number of innovations along the same line or by historical circumstances, such as political borders of long standing, or by migrations that have brought into contact two populations whose dialects were developed in noncontiguous areas. (Source: Britannica)

English and gender podcast (University of Oxford)

We gain overt prestige when we're looking towards the dominant culture.

We gain covert prestige when we're looking towards a subgroup or subversive culture.

Covert - ethnolectal varieties of Australian English eg Greek English, Chinese English.

Covert  - Surfing slang, BAA, slang Vocal Fry. It verges on jargon. Occupational jargon as a marker of groups. 

Lawyers' jargon/legalese - is it covert or overt? It uses standard Australian English (overt) as a marker, plus the legal jargon for covert prestige. It's like a hyperstandard (so specialised that it's signifying being a member of a group.)