Could anybody explain the etymology of the phrase broth of a boy? I know the meaning but cannot understand how it happens that it means what it means.
Broth of a boy, from page 178 of the Concise Oxford English Dictionary: Luxury Edition:
Irish a very lively boy or young man.
ORIGIN OE, of Gmc origin; rel. to BREW
Broth of a boy is thought to be of Irish origin, not directly from English broth, but probably from a similar linguistic stock:
Old English broþ, from Proto-Germanic *bruthan (cognates: Old High German *brod),
from verb root *bhreue- "to heat, boil, bubble; liquid in which something has been boiled" (source also of Old English breowan "to brew;" see brew (v.)).
Picked up from Germanic by the Romanic and Celtic languages.
The Irishism broth of a boy, which is in Byron, was "thought to originate from the Irish Broth, passion -- Brotha passionate, spirited ..." [Farmer], and if so is not immediately related.
Old English breowan "to brew" (class II strong verb, past tense breaw, past participle browen),
from Proto-Germanic *breuwan "to brew" (cognates: Old Norse brugga, Old Frisian briuwa, Middle Dutch brouwen, Old High German briuwan, German brauen "to brew"),
from PIE root *bhreuə- "to bubble, boil, effervesce" (cognates: Sanskrit bhurnih "violent, passionate," Greek phrear "well, spring, cistern," Latin fervere "to boil, foam," Thracian Greek brytos "fermented liquor made from barley," Russian bruja "current," Old Irish bruth "heat;" Old English beorma "yeast;" Old High German brato "roast meat"),
the original sense thus being "make a drink by boiling."
Etymonline.com Emphasis mine