Green’s Dictionary of Slang

mountain n.

1. (US black) in pl., large, noticeable female breasts.

[[UK]Harris’s List of Covent-Garden Ladies 55: Flow’ry mountains, / Mossy fountains, / Shady woods, / Christal floods, / With wild variety surprize].
[[UK]Peeping Tom (London) 1 2/2: Two snowy mountains rise above / [...] / Which Venus named the Hills of Love].
[US]R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].

2. (US campus) an erection.

[US]College Sl. Research Project (Cal. State Poly. Uni., Pomona) 🌐 Mountain {somewhat vulgar}(noun) A boner; an erection.

3. see mountain dew n.

SE in slang uses

In compounds

mountain canary (n.) [joc. ref. to its braying]

(US, mainly Western) a donkey.

[US]DN VII 114: Mountain canary [...] Burro.
Almirall College to Cow Country 406: Mountain Canaries and the Rustler [HDAS].
mountain climber (n.) [play on high adj.1 (3)]

(US campus) a feeling of intoxication produced by drugs.

[US]Eble Campus Sl. Apr. 3: mountain climber – a high induced by drugs.
[US]Eble Sl. and Sociability 31: Often the second part of a noun + noun compound is derived from a verb by the suffix -er: [...] mountain climber ‘high induced by drugs’.
mountain devil (n.) [the nickname for the thorn-devil (Moloch horridus), a Tasmanian lizard]

(Aus.) a native of Tasmania.

[[Aus]Launceston Examiner (Tas.) 10 June 5/4: Fiji [...] The Kai Colos, or ‘Mountain Devils,’ have been giving some trouble again [...] They came down and attacked and burned several native villages].
[Aus]Baker Popular Dict. Aus. Sl. (2nd edn).
[Aus]Chron. (Adelaide) 24 Mar. 43/3: For Tasmanians, Van Diemenese, Derveners, Derwent Ducks, Mountain Devils, Tassies and Tassylanders.
mountain dew (n.)

see separate entry.

mountain goat (n.)

(US prison) any form of prison meat.

[US]N. Algren Man with the Golden Arm 208: ‘Wait till we get that mountain goat’ — warless soldiers [...] indifferent to Sunday mutton.
[US]Goldin et al. DAUL 142/1: Mountain goat. (P) Sliced mutton or roast beef.
mountain guinea (n.) [guinea n.1 ]

(US/Ital.) used by southern Italians, a northern Italian.

[US] oral testimony in Lighter HDAS II.
[US]H. Gould Cocktail 259: They think I’m a mountain guinea.
(con. 1930s) P. Gropman Glorious: A Story of the CCC 12: ‘C’mon mountain guinea. Time’s a’wastin’.’ They walked down the path to the motor pool.
J. Tuccille Heretic: Confessions of an Ex-Catholic Rebel 148: Coach Fox threw up his hands in disgust. ‘You’re hopeless,’ he said. ‘I thought you’d be breaking two minutes with that other mountain guinea.’.
mountain pecker (n.)

1. a sheep’s head.

[UK]Hereford Jrnl 1 Feb. 4/5: Vy, everybody knows [...] a mountain pecker is — lor bless me [...] vy, a sheep’s head.
[UK]Hotten Dict. of Modern Sl. etc.
[US]Letters by an Odd Boy 165: Some of the slang expressions, also, are simply funny; as, for example, when you call a sheep’s head ‘a mountain pecker’.
[UK]Sl. Dict.
[UK]Sporting Times 29 Mar. 2/1: The sheep’s-jimmies (or ‘mountain peckers’) were superb, whilst the ’74 Castle U.P. was a poem.

2. a Welshman.

[UK]D.W. Barrett Life and Work among Navvies 51: Why a Welshman should be termed a ‘Mountain-pecker’ [...] I must leave my readers to guess.