Other forms: allied, allies, allying
An ally is a country or person on your side. A close ally, allies in the war, find an ally.
Between states it is a formal relationship with obligations, and the Allies with a capital letter names the coalitions of the two world wars. In modern social use an ally is somebody who supports a group they do not belong to. Note that the stress differs: the noun is ALL-y and the verb is a-LY.
1nounallyA country or person on your side.
“Britain and France were allies in the First World War.”
2verbjoinTo join with somebody in support.
“The small country allied itself with its powerful neighbour.”
Too few real examples of this word to estimate which sense is commonest.
ally is a supporter.
Old French alier comes from Latin alligare, to bind to — ad plus ligare, to bind. Alliance, ligament and league come from the same binding.
Proto-Indo-European
*leyǵ- — to bind, tie
Latin
alligāre
Old French
alier — to join together, unite; to alloy (metals); (cooking) to combine ingredients
Middle French
alier
Anglo-Norman
alier
Middle English
allien
English, today
ally
Built from
From Middle English allien, alien (“to form an alliance, associate, join; to become an ally; to introduce (someone) as an ally; to marry; to become related (to someone); to attack, engage in combat; to combine; (cooking) to combine ingredients, especially to bind them together”)alie, alleye, allie, ally, allye, aly, alye, alyȝe, from Anglo-Norman alier, allier, Middle French alier, allierallyer, alyer, and Old French alier (“to join together, unite; to alloy (metals); (cooking) to combine ingredients”) (modern French allier), from Latin alligāre, the present active infinitive of alligō, adligō (“to bind around, to, or up (something), bandage, fasten, fetter, tie; to hold fast; to detain, hinder”), from al-, ad- + ligō (“to bind, tie; to bandage, wrap around; to unite”) (from Proto-Indo-European leyǵ- (“to bind, tie”)). a doublet of allay and alligate and alloy and ligament.
Etymology from Wiktionary, CC BY-SA