Green’s Dictionary of Slang

happy as... adj.

[mid-19C+] in phrs. meaning either very happy or very unhappy.

In phrases

...a bastard on father’s day

[1950s+] (Aus.) very unhappy.

...a black in a barrel of treacle

[1900s] extremely happy.

...a boxing kangaroo in fog time

[20C+] (Aus.) very discontented, very unhappy.

...(a box of) birds [...the chirping of birds]

[late 19C+] (Aus./UK/US) in very high spirits.

...a bug in a rug (also ...a bug on a hot stove, ...a bedbug/June bug, lively as bugs in a rug)

[1910s+] (US) very happy.

...a clam (also ...a clam at high tide/water, ...a cricket, ...a horned toad, ...a sandbag)

[mid-19C+] (US) very happy, totally satisfied.

...a dog with two tails (also ...with a tin tail, ...with two dicks/mickeys, ...with a bellyful of soup and a streetful of lamp-posts)

[1950s+] very happy.

...a flea at a dog-show (also ...as moths in a best blanket)

[20C+] (N.Z.) very happy.

...a hophead

[1940s] (US) very happy.

...a nun weeding the asparagus

[1910s+] (Can.) very cheerful (with obvious sexual overtones given the ‘phallic’ asparagus).

...a pig in shit (also ...pigs in shit, ...a pig in clover, ...muck, ...mud, ...shite, ...sugar, ...hogs in shit, ...pigs in mud)

[19C+] extremely happy.

...a sick eel on a sandpit (also ...a hooked fish)

[1940s] (N.Z.) very unhappy.

...Larry (also ...Harry) [ety. unknown; ‘possibly but not certainly commemorating the noted Australian pugilist Larry Foley (1847–1917)’ (Baker, 1966); more likely a link to larry n.2 ]

[late 19C+] (orig. Aus.) perfectly happy, quite content.