What is the entomology of "ligger"?

It appears to come from a dialectal variation of the verb to lie: to idle or lie about:

Ligger:

• ‘Hangers on’ such as ‘music groupies’ for LIGGERS is an example of what it can mean, but it’s not the whole story.

The Oxford English Dictionary provided the following:

• LIGGER noun [from verb ‘lig,’ + ‘-er’]: One who gatecrashes parties, a ‘free-loader.’

• LIG verb [from dialect variation of the verb ‘lie’]: To idle or lie about (colloquial); also (slang), to sponge, to ‘freeload’; to gatecrash or attend parties. LIE verb: To be in a prostrate or recumbent position.

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Cassell’s Dictionary of Slang, however, isn’t so sure of the above derivation and offers some additional possibilities:

• LIGGER noun (also LIG) [1960s and still in use]: A hanger on, especially, in show business, a ‘freeloader.’

• Etymology debatable; either acronym of least important guest; or Standard English linger, to hang around; or Banffshire dialect lig, to gossip, to talk too much. Most likely it is dialect lig, to lie around. The term became widespread in the early 1970s, but dates at least to 1960 when Colin MacInnes (1914-76) used it in an essay on poncing (Britishism for pimping) – ‘The Other Man’.

An alternative origin is suggested in Brit Slang, by Ray Puxley, (2003):

• A theatrical term from the 1960s when a gatecrasher or uninvited guest became a 'ligger'. Someone who likes to be seen in the company of the rich and famous, a hanger-on. Related to 'lig' (qv), which may be an acronym of Least Important Guest.

As for its usage, it appears to be still current: (from the Oxford Learner's Dictionary)

Ligger:

• person who always takes the opportunity to go to a free party or event that is arranged by a company to advertise its products.

  • a roomful of liggers drinking free champagne

To supplement Josh61's answer, here (in chronological order) are four discussions not mentioned in the body of his answer. From Jonathon Green, The Dictionary of Contemporary Slang (1984):

ligger n. a hanger on; spec. in entertainment industry: a freeloader (qv). fr. least important guest (?) or linger: hang around N[ew] M[usical] E[xpress].

From Paule Beale, Partridge's Concise Dictionary of Slang and Unconventional English (1989):

lig, n. and v. A freeloader, freeloading (ligger, ligging); one who takes, taking, the advantage of free drinks: 'pop' music, media, journalistic: since earlier 1980s. (John Ryle.)

From Tony Thorne, The Dictionary of Contemporary Slang (1990):

lig vb British to freeload, enjoy oneself at someone else's expense. The word, coined in Britain in the early 1970s, refers to the activities of hangers-on, groupies, music journalists, etc., who attend receptions, parties, concerts, and other functions, usually financed by record companies. The origin of the word is obscure, it has been suggested that it is made up of the initials of 'least important guest' or is a blend of linger and gig. Alternatively it may be an obscure vagrants' term from a dialect survival of Anglo-Saxon liegan, to lie.

lig n British an opportunity for freeloading, a party, reception or other occasion when it is possible to enjoy oneself at someone else's expense. The word refers to the rock and pop-music world, and probably postdates the verb form lig and the noun ligger.

...

ligger n British a freeloader, hanger-on or gatecrasher at concerts, receptions, parties, etc., in the rock and pop-music milieus. The word is part of rock music's jargon and was adopted enthusiastically by journalists in such publications as New Musical Express in the 1970s to describe those enjoying themselves at the expense of record companies. [Citation:] 'Julia Riddiough, 27 "going on 180", is a world-class ligger who could club for Britain.' (Observer, Section 5 magazine, 7 May 1989).

From John Ayto & John Simpson, The Oxford Dictionary of Modern Slang 1992):

lig verb intr. 1 To loaf about. 1960–. IT It's a time for ligging in the streets and doing your thing, man (1969). 2 to freeload, esp. by gatecrashing parties. 1981–. RADIO TIMES [I] suddenly twigged what ligging was all about when I got my first job as a researcher on Aquarius I found ... I could get free tickets for everything, everywhere (1985). {From dialectal variant of lie verb, to repose.}

ligger noun One who gatecrashes parties, a freeloader. 1977–. OBSERVER I went to a party Wednesday that was a liggers' delight (1985). {From LIG verb + -er.}

The etymological explanations in these dictionaries seem to get more arcane as they get farther from the period when the slang term first caught on. Still, one older source does report the 19th-century existence of the dialect form lig. From C. Clough Robinson, The Dialect of Leeds and Its Neighbourhood (1862):

LIG. To lie. "Ah gottant t' hēad-wark bad." Awāay wi' thuh, lig thuh darn a bit then, an it'll happen goa awāay." "Ligging ont' grund thear." "Ligging i' bed an duing nowt." "It's liggen ont' floor long eniff nah, tak it up." "What's tuh liggen on't?"—what have you laid, or bet, on it? "Av liggen ten shillin' darn."

It's possible, I suppose, that a freeloader from Leeds crashed an EMI release party sometime in the 1970s, drank too much, announced that he had a "hēad-wark" and was going to "lig darn" for a while—and gave the term ligging new life on a national scale. But I'm not entirely convinced.

I come from Lancashire, a county in the north west of England and am a keen amateur hedgelayer. When hedgelayers pleach a stem by partially cutting through it and lay it over to one side it is called a pleacher in most parts of the country. However here in Lancashire it is called a ligger. I presume from the Anglo Saxon origin meaning to lie down.