Act I

let the pirate bumper pass – a bumper is a cup or glass filled to the brim, as for a toast

scuttling a Cunarder – sinking a passenger ship of the Cunard line

cutting out a White Star – separating a passenger ship of the White Star line from the surrounding ships in order to capture it (the Titanic was a ship of the White Star line)

on breakers always steering – always making mistakes and getting into trouble; as when steering a ship toward waves breaking on rocks

can it be Custom House – customs enforcement patrol, like our Coast Guard

the glass is rising very high – weather-glass: a barometer (rising indicates fair weather)

your pirate caravanserai – as “caravanserai” is a sort of inn for caravans and their occupants, the only excuse for applying this word to a group of men it that it rhymes with Chancery (almost)

Wards in Chancery – minors under the protection of the Court of Chancery

from Marathon to Waterloo – in 490 BC. the Greeks defeated the Persians at Marathon (the messenger who died after running the 26 miles to Athens with the news somehow inspired modem “marathons”).  Napoleon’s final defeat was at Waterloo in 1815.

the scientific names of beings animalculous – an animalcule is a microscopic animal

I answer hard acrostics – a parlor game similar to charades, with acted-out words, whose first letters then spell out the real message to be discovered.

quote in elegiacs all the crimes of Heliogabolus – even the use of a verse form (elegiacs) could not soften the awful deeds of this most appalling Roman emperor (212-222 AD)

In conics I can floor peculiarities parabolous – conics is the study of geometric properties of a cone cut by imaginary planes, producing parabolas, ellipses, and hyperbolas. To floor is to defeat (as in wrestling). Parabolous is Gilbert’s adjective variant of parabolic.

I can tell undoubted Raphaels from Gerard Dows and Zoffanies – three painters, from three different centuries and countries, and with quite distinct styles.

the croaking chorus from “The Frogs” of Aristophanes – a comedy produced in Athens in 405 B.C. (the croaking chorus goes “Berkekekex, koax, koax”)

a washing-bill in Babylonic cuneiform – a laundry-list, in ancient wedge-shaped writing

ev’ry detail of Caractacus’s uniform – this Welsh king who resisted the Roman invasion of Britain had a limited uniform:  it consisted of woad, a blue dye… and nothing else!

whistle all the airs from that infernal nonsense Pinafore – H.M.S. Pinafore was the Gilbert & Sullivan show which preceded The Pirates of Penzance.  It was their first great success

mamelon and ravelin – terms for strategic earthworks: mamelon is a mound used in fortifications, ravelin is a sort of ridge

such affairs as sorties and surprises – sudden troop movement outward when besieged

and when I know precisely what is meant by commissariat – the provisioning branch of the Army

has never sat a gee – never ridden a horse (“gee-gee” being a childish way of referring to a horse, derived from a word of command to horses)

Divine Emollient! – something that softens, as poetry apparently does, even for pirates

Pray observe the magnanimity we display to lace and dimity – a thin cotton fabric.

Act II

dishonor on the family escutcheon – shield displaying heraldic insignia; family crest

threatened with emeutes – a French term for riots or brawls

when the coster’s finished jumping on his mother – costermongers (street vendors of fruit, fish, etc.) were sometimes rather rough characters

life preserver – a stick or bludgeon loaded with lead, intended for self-defense, but all too often used by evil-doers (as in this case)

unshriven, unannealed – without having made confession or having received extreme unction

with humbled mien – manner, or general bearing

we love our House of Peers – the House of Lords, one of the two Houses of Parliament.  A peer has at least one of the following titles:  duke, marquis, earl, viscount, and baron.