Origins
Panini is the Italian plural of panino, a diminutive of pane (bread). In Italy, a panino is simply a bread roll with a filling — nothing more specific than that. It can be cold or warm, pressed or not, made with any type of bread. The word carries no implication of grilling or pressing. Asking for a panini in Milan gets you a sandwich in a roll; asking for one in London or New York gets you something pressed flat and marked with grill lines.
The Latin ancestor is panis, bread, from the Proto-Indo-European root *pa- meaning to feed or protect. This root generated a remarkable number of English words through different channels: pantry (a bread storage room), companion (one who shares bread), company (a group sharing bread), and pastor (one who feeds a flock). All connect back to the elemental concept of feeding.
Italian bread rolls called panini have existed for centuries, but the specific grilled-and-pressed version familiar to English speakers developed in the 1970s and 1980s. Italian cafes in Milan began using electric sandwich presses in the 1970s for quick heated sandwiches. The trend spread to the United States and Britain in the 1990s, where panini became a staple of cafe menus.
Later History
English adopted the plural form as a singular noun — one panini, two paninis. This double-plural construction (Italian plural plus English plural -s) is grammatically redundant but linguistically normal. English did the same with other Italian borrowings: one graffiti rather than one graffito, confetti treated as singular mass noun rather than plural.
The panini press itself became a popular home kitchen appliance in the 2000s, cementing the word's association with a specific cooking method that has little to do with its original Italian meaning of small bread.