We've discussed food in several other threads here, so I'd like to open a new one on a yet seldom covered but interesting sub-category, usually partaken of while walking or lounging about.
Mostly it comprises food originating in the 1800s or earlier as cheap, quick fare for poor city workers or country peasants, who could not spare much time/money on their lunch breaks. Therefore, their original ingredients were mostly 'leftovers' from costlier, unaffordable alternatives, usually fried to mask their originally unappealing flavour - i.e. 'humble' food often eaten hurriedly, standing up or barely seated - but becoming more fashionable today to time-harassed office workers

.
I shall let others dwell on better known items such as:
- US hot dogs & hamburgers
- Turkish doner kebap/Arabic kebab/Greek souvlaki
- English fish & chips (interestingly of Spanish Jewish origin, the
pescal)
- Italian gelato (probably more of a dessert than an
impromptu meal)
Rather, I'll mention below a few, lesser known Italian regional items (north to south):
ROMAGNA (Rimini)Piadina: flat bread typically stuffed with ham.
TOSCANA (Florence)Lampredotto: a sandwich made made with the 4th and final stomach of a cow - the
abomasum - vaguely resembling the meat of
lamprey eels.
LAZIO (Rome)Supplì: a fried meat ball with mozzarella inside, supposedly derived from
surprise when French troops in Rome ((1850s) saw the cheese tendrils emanating from a bite into it.
ABRUZZO (Ascoli)Oliva ascolana: a large, fried meat-stuffed olive.
PUGLIA (Bari)Polpo arricciato: octopus first beaten on the quay or a rock to soften then 'curl up' its rubbery tentacles (our Pugliesi are extremely fond of fresh, raw sea dwellers, preferably shelled).
CALABRIA (Catanzaro)-
Morzeddhu (small morsel): Calabria probably has the hottest Italian cuisine (they make liberal use of chili), and this is basically a hot sandwich of veal entrails.
SICILIA-
Arancinu (small orange): fried rice ball coated with breadcrumbs, variously stuffed.
P.S.: one could argue that also
pizza and
bruschetta have a similar origin, but that is not certain.